Chapter 8: Heredity

The material in this chapter will help you solve genetics problems.

A genetics problem is an analysis of the characteristics of parents and offspring.
You have to determine the traits of the other generation based on the traits of one generation.

The application of probability rules is required for genetics problems.

There is a chance that the coin will be heads.
If a coin is thrown again, there is a chance that it will be heads.
The first toss does not affect the second toss.

You simply add the probabilities of each event happening separately to determine the probability of two or more independent events.

The probability of getting two heads is 1/2 x 1/2.
The probability of three heads is 1/2 x 1/2 x 1/2.

The formula for carrying out these instructions is described by a genetic code.

Pea plants have a gene that codes for stem length.

There are two alleles of the gene for stem length in pea plants, one for tall plants and one for dwarf plants.

Every gene has a unique location.

One of the chromosomes was contributed by each parent.

The stem length on one pea plant's chromosomes might be different from the stem length on the second parent's chromosomes.

The actual trait is expressed by the dominant allele.

The tall stem allele is dominant in pea plants.
If a pea plant has one of the alleles, the dominant one, it will produce tall plants.
A dominant allele is represented by a capital letter, while a recessive allele is represented by a lower form of the same letter.
The first letter of the dominant allele is often used to represent the gene.

The dominant trait is expressed.

It is normal to write a pair of alleles with the dominant one first.
Only the dominant allele is expressed in this condition.

The tall stems, blue eyes, and brown hair all have the same allele.

The term geno is used to describe all of an individual's alleles, and its phenotype is used to describe the expression of those alleles.

The laws of segregation and independent assortment were discovered by a nineteenth-century monk.

The laws describe the separation of chromosomes.
The rules of probability can be used to describe how the different chromosomes are assembled in offspring.

Each gamete contains only one copy of each of the two chromosomes, so that one member of each pair migrates to the opposite pole.

The process is not influenced by the migration of genes within one pair of chromosomes to opposite poles.

A cross between a tall pea plant and a dwarf pea plant is a mono hybrid cross because it is investigating a gene for only one trait.

A cross looking at the stem length and flower color is a di hybrid cross.

A cross investigating seed color and texture is another example.

The first thing to do in analyzing the genetics problem is to figure out which parents produced which gametes.

The law of segregation states that each chromosome migrates to an opposite pole and ends up in a separate gamete.

The next step is to figure out how the gametes can combine.

The easiest way to accomplish this is to create a Punnett square.

There are four boxes in the middle that combine the allele found at the top with the one found to the left.

The possibilities of combining the two gametes from one parent with the two gametes from the second parent are represented by the four boxes.
The genotypic frequencies of the offspring are represented by these results.
The plants are 1/2 tall and 1/2 dwarf.
Results can be given as frequencies, percents, or ratios.
The dwarf to tall plants ratio is 2:1.

The alleles of all possible gametes are the first step in analyzing this cross.

The law of segregation says that one allele of each pair migrates to opposite poles and ends up in separate gametes.
The next step is to figure out how the gametes from one and the other parent can be combined.

In the 16 boxes of the square, the gametes at the top and left are combined.

The number of times each genotype appears is the next step.
The final step is to count how many times each phenotype appears.
Some of the phenotypes have more than one genotype.
There are nine plants with yellow and round seeds, three plants with green and round seeds, three plants with yellow and wrinkled seeds, and one plant with green and wrinkled seeds in the F2 progeny.
The same ratio was observed in his experiments for this cross.

You want to know the genetics of a dwarf pea plant.

You want to know the genetics of a plant with tall stems.

A test cross is used to determine which genotype is correct.

You will always be aware of the individual's genetic makeup.
If you don't know the second allele for the first individual, leave a blank space for it with an underscore.

There is an analogous situation presented by a coin toss.

Half the time it will be tails if you toss a coin six times.
There is a small chance that all six tosses will be tails.

There are times when the alleles for a gene don't exhibit the dominant and recessive behaviors.

Sometimes both alleles are written with the same letter but with a different number or letter to differentiate them.
There may be no apparent rationale for the method used to indicate different alleles.

Both inherited alleles are expressed in this pattern.

The M and N blood types produce substances on the surface of human red blood cells.

Imagine a continuum to help you distinguish inheritance.

There is complete dominance by a dominant allele.

Both alleles are expressed at the other extreme.
A blend of two different alleles produces an intermediate phenotype.

The A, B,AB, and O types correspond to the presence or absence of an A or B sugar component in red blood cells.

Half of the A sugar is attached and the other half is attached to the B sugar.

Blood transfusions must be made between people with similar characteristics.

O type blood contains no A or B sugars, so anyone can accept it.

A person with O type blood is a universal donor for the ABO blood group.

Yellow and green pea seeds and A, B, and O blood types are examples of a range of varieties.

The law of independent assortment states that if two genes are on different chromosomes, they should be separated.

The law of independent assortment states that genes that are linked do not obey it.

The body color and wing structure are affected by two of these genes.

About 18% of the time, linked genes cross over during prophase I.

The more places between the genes that the chromosomes can break, the more likely the two genes will cross over during synapsis.

Recombination frequencies are used to give a picture of the arrangement of genes.

Animals have one pair of chromosomes that do not have the same genes.

In mammals and fruit flies, there are two X chromosomes that make up a female and one X and one Y that make up a male.

The Y chromosomes have relatively few genes and are small compared to the X.

There are additional considerations when working with sex linked genes.

Two copies of the sex-linked gene are given to females when they inherit a sex-linked genes.

This is the same situation as for autosomal inheritance.
A male will only inherit one copy of the gene if he has the X chromosome.

The Y chromosome does not deliver a similar gene.

The X chromosome of a male is the only one that expresses his trait regardless of whether it is dominant or not.

Color blindness, as well as other sex-linked genetic defects, are more common in males.

Female mammals have two X chromosomes in each cell, but one of them does not leave the nucleus of the cell.

Most of the genes are not expressed nor do they interact with the X chromosomes that are expressed.
Only the genes on the one active X chromosome are expressed by that cell.
One of the two chromosomes in each embryo becomes inactive when X-inactivation begins.
The X chromosomes from the progenitor cell will be inactivated as will the X chromosomes from the subsequent daughter cells.
Some groups of cells will have one X chromosomes inactivated, while other groups will have the other X chromosomes.
All of the cells in a mammal are not the same.

A very visible example of X-inactivation can be seen in the different groups of cells that produce different patches of color.

The cats have hair that is randomly arranged over their bodies.

The orange and black colors are determined by a gene on the X chromosomes.
The hair is black when the X chromosome with the orange allele is inactivated.
There are patches with the black allele that are orange.

Gametes with extra or missing chromosomes can be produced by the failure to separate two or more chromosomes.

During anaphase, the failure of two chromatids of a single chromosomes causes daughter cells with extra or missing chromosomes.

If a polyploid gamete is fertilized with a similar gamete, a polyploidy zygote can be formed.

Plants have polyploidy.

Gene function can be affected by most point mutations.

When lowoxygen conditions occur, the red blood cell becomes sickle shaped because of the defects in the hemoglobin molecule.

Red blood cells don't flow through the capillaries freely and oxygen isn't delivered throughout the body.

Brain cells die when these fats accumulate in the nerve cells of the brain.

Nondisjunction is the most common cause.

Most aneuploid gametes do not produce viable offspring, but some do.
These can lead to genetic disorders.

Down syndrome individuals have many defects, including mental retardation, heart defects, respiratory problems, and deformities in external features.

There are either sex chromosomes or no sex chromosomes in the embryo.

sterile females with physical abnormality are the ones who have eggs.
The absence of the Y chromosome, with its few malenecessary genes, is not nearly as harmful as the absence of a single chromosomes.

These individuals may be sterile because of the presence of a Y chromosome.

A variety of female secondary sex characteristics can be mildly expressed by individuals.
The Barr body is formed by the extra X chromosome, but it is mostly inactive.
When an XXX egg combines with a normal X sperm, these trisomic X females are usually without serious disorders, though they are often tall and may have some learning disabilities.

If the inversion does not introduce any deletions or duplications, individuals with inversions will not express any abnormality.

Down syndrome usually occurs when an individual has an extra chromosome 21.

Down syndrome can also occur after a chromosome segment is changed.
An individual with a 14-21 translocation has a normal number of chromosomes but has three copies of a segment from the other side of the chromosomes.
The result has the same effect.

If you have ever compared identical twins, you know that they are not the same.

Environmental factors influence the expression of their genes.
The environments in the uterus of identical twins can be slightly different, and this can lead to differences in height, weight, and fingerprints.

Physical development in animals and plants can be influenced by nutrition.

Plants growing in soils that lack adequate amounts of nitrogen may not flower or produce smaller than normal fruits due to insufficient calcium or other deficiencies.

The expression of genetic disorders may be influenced by nutrition.

Individuals with phenylketonuria can't metabolize phenylalanine.
Brain cells die and phenyl accumulates as a result.
The amount of phenylalanine in the body remains safe if the diet is reduced to minimal levels.

Eliminating dairy products containing lactose from a person's diet can help them avoid symptoms of nausea, gas, and diarrhea.

Eggs at lower temperatures become males, while those at higher temperatures become females.

The color of animal fur can be affected by temperature.

The genes for fur color in Siamese cats and Himalayan rabbits produce a dark fur color in the cold areas of the animal and a light fur color in the warm areas.

Increased exposure to the sun's UV radiation causes an increase in melanin, the skindarkening pigment.

The flower color is blue when the soil is acidic and pink when it is basic.

Chemicals in the environment produced by other individuals can affect the expression of one individual's genes.

This kind of chemical signaling between organisms is required to have sex.

When there is not enough food, slimebacteria make signaling molecule that stimulates nearbybacteria to aggregate, form a multicellular collective, and grow into a fruiting body.
Certain yeast cells only mate with yeast cells of the opposite sex.
To signal its presence, a yeast cell makes a signaling molecule and only yeasts of the opposite sex respond.

Mitochondria and chloroplasts have their own genes that play a role in their metabolism.

The male gamete (pollen or sperm) delivers very little cytoplasm in most plants and animals.
One example of maternal inheritance is a genetic variation that affects nerve function.
The trait can only be passed on from mothers with defects in the mitochondria.

A review of the material presented in this chapter is provided by the questions that follow.

They can be used to evaluate how well you understand the concepts.
AP multiple-choice questions are often more general, covering a broad range of concepts.
The two practice exams in this book are for these types of questions.

Four possible answers or sentence completions are followed by each of the following questions or statements.

The one best answer or sentence is what you choose.

The ability to taste a chemical is an inherited trait.

Dumpy wings in fruit flies are larger than normal wings.

300 normalwinged and 100 dumpy-winged flies were produced by two normal-winged flies.

Each answer in the key may be used once, more than once, or not at all.

The genes that produce normal wings and bristles are wild-types.

People with red hair often have freckles.

The genes for the two traits are the same.

The genes for these two are related.

Both of these traits are codominant.

Both parents have red hair.

The two genes are related.

The two chromosomes are related.

There are circles and squares for males and females.

The male and female are connected by a horizontal line.
In this case, red-green color blindness, a filled circle or filled square indicates that the individual has a particular trait.
There is a sexlinked, recessive allele that leads to color blindness.

The next three questions will be answered using the following key.

The answer in the key can be used more than once.

The geno for 10 is identified.

The genotype for individual 5 is identified.

Do you know the genotype for individual 14?

Cats can be either orange or black, or they can be a coat of black and orange.

A black female and an orange male can have a cat.

A black female and an orange male can have a cat.

A male and female can produce a cat.

A female and male can produce a male cat.

The genes for seed color and flower color are located on the same chromosomes in peas.

If there is a cross between a plant with both traits and a plant with only one, the following offspring will be produced.

The questions that follow are typical of an entire AP exam question or just that part of a question that is related to this chapter.

There are two types of questions on the AP exam.

It takes about 20 minutes to answer a long free-response question.
Sometimes they offer you a choice of questions to answer.
6 minutes is the time it takes to answer a short free-response question.
diagrams can be used to supplement your answers, but a diagram alone is not adequate.

Color blindness is an X-linked trait.

Explain why males have a greater chance of being color blind.

There is a form of resistance to the sun's rays.

In one or two sentences, explain why females are more likely to express the vitamin D-resistant trait.

A person's height can be any value over a wide range.

There is a mechanism that can produce variation.

Turner syndrome will turn up a three if the probability of each event happening independently is 1/ x 1/.

The various forms of a gene are referred to as alleles.

There are two forms of a single gene that code for seed texture that are referred to as Wrinkled and Round.
Answer choice A is incorrect because wrinkled and yellow refer to two different genes, one for seed texture and the other for seed color.

If there is no crossing over, the two chromatids will migrate to opposite poles and become separate gametes.

The sheep has a black color.

That was given to you in the question.
There is no single parent that has a single genotype.
Although one parent is black, you cannot be certain if the other is.

There is a long way and a short way to solve this problem.

A Punnett square would be needed to sort and count all the offspring.
No offspring will be white flowered since this is not the case.

It's not usually practical to make a Punnett square for more than two genes.

Look at the genes separately to solve the problem.
Find the product of the frequencies.

The range of skin colors in humans is most likely code by many genes.

Two traits are likely to be linked when they occur frequently.

A freckled person with red hair may not have freckles.
In these cases, there was an event where one of the genes swapped with the other genes that didn't code for red hair.

The false statement is answer choice C. Answer choice B is true because the same genes would be on the homologous chromosomes.

Begin by drawing a horizontal line.

Both positions are possible at this point.

If you want to solve a problem, you should start by identifying the only possible ones.

For a sex-linked pattern of inheritance, you can identify the different types of males and females that have the same trait.

The next step is to identify the normal females.

Try to identify every female.

To be a calico, a cat must have two X chromosomes, one orange and one black.

Since a male cat only has one X chromosomes, it can be either orange or black.

Turner syndrome can be caused by the nondisjunction of the sex chromosomes.

A sperm or egg missing a sex chromosomes is a result.
It is possible to form an OY zygote, but it is not viable because it is missing an X chromosomes.

Down syndrome is caused by nondisjunction but not the sex chromosomes.

There are two types of inherited diseases, one of which is a sex-linked recessive allele.

There are few places where the chromosomes can break and cross over if the two genes are very close together.

There are many places for chromosome breaks if the genes are far apart.
When the genes are so far apart that they cross over frequently, the genes seem to be on different chromosomes, as if they were on different chromosomes.
That is what has happened in this question.
The frequencies would have been expected if the genes were on different chromosomes.
Since the question states that the genes are linked, they must be far apart to allow so many.

Only two kinds of progeny phenotypes are produced and they would be produced in equal numbers without crossing over.

The table has the largest numbers, and .
The smaller numbers represent the offspring that are produced as a result of crossing over.
The total number of offspring produced is 166.
The total number of offspring is 77 + 88 + 683, or 1,500.

All the genes on the X chromosomes are expressed by males regardless of whether they are dominant or not.

In order for a female to express a trait, she must have two X chromosomes and two alleles for the same gene.

Because females have two X chromosomes, they can inherit a trait on one of them and only need one copy of a dominant allele to express it.

Continuous variation in a population is caused by a trait that is produced by the expression of many interacting genes.

Many genotypic and phenotypic combinations can be produced by multiple alleles of a single gene.

During meiosis I, the chromosomes align on the metaphase plate.

Each homologue migrates to a different pole and becomes a different gamete.
The migration to separate poles is random because either of the chromosomes can migrate to either pole.

There are different pairs of chromosomes.

The law of independent assortment states that genes that are on different chromosomes migrate independently of genes on other chromosomes.

Two genes are on the same chromosomes.

Unless crossing over occurs, they migrate to either pole if they are on the same chromosomes.
They violate the law of independent assortment by being a single gene in a mono hybrid cross.
The genotypic and phenotypic ratios are expected from a typical mono hybrid cross.

There is crossing over between genes.

As the distance between the genes increases, the crossing over increases.

Sex-linkage occurs when a gene is located on one of the sex chromosomes.

In humans, there is an allele on the X chromosomes that causes hemophilia.

Females have two copies of the gene on their X chromosomes.

They are a type of bleeding disorders if they receive two all genes.
If they inherit one of the two normal all genes, they will have normal clotting abilities but will be carriers of the disease.
They will have normal clotting if they receive the normal allele.
Sex-linked diseases are more common in males than in females because they only need one copy of the allele to express the trait.

Down syndrome is caused by the nondisjunction of the two number 21 chromosomes.

The law of segregation states that the homologous pair does not separate and move to opposite poles, but rather they end up at the same pole and gamete.
There are two kinds of gametes, one with two copies and one without.
Only two copies of the chromosomes are viable.
There are three copies of chromosomes 21 in the zygote formed between this gamete and a normal gamete.

This results in a gamete with either two sex chromosomes or no sex chromosomes.

If a gamete with no sex chromosomes is combined with a normal gamete, the result will be a single X chromosomes and Turner syndrome.
Turner syndrome individuals are female and have physical defects.