The chromosomal foundation of Mendel's rules. In this paper, we compare the outcomes of one of Mendel's dihybrid crossings (as shown in the image attached) to the behavior of chromosomes during meiosis.

The separate assortment and segregation of the alleles for seed color and shape are accounted for by the arrangement of chromosomes during metaphase I of meiosis and their movement during anaphase I, respectively. In an F1 plant, each cell that goes through meiosis generates two types of gametes.

When all cells are counted, each F1 plant generates an equal amount of all four types of gametes because of the various chromosomal configurations during metaphase I am equally likely.

The separation of homologs during anaphase I provide for the segregation of a gene's two alleles into distinct gametes, and the random arrangement of chromosomal pairs during metaphase I account for the independent assortment of alleles for two or more genes situated on different homolog pairs.

This picture depicts the same dihybrid pea cross that you saw in Figure 14.8.

The image attached depicts how the behavior of chromosomes during meiosis in the F1 generation, followed by random fertilization, results in the F2 phenotypic ratio reported by Mendel.

Another benefit of the fruit fly is that it only has four pairs of chromosomes, which are easily distinguished using a light microscope.

There are three autosome pairs and one pair of sex chromosomes. Male fruit flies have one X chromosome and one Y chromosome, whereas females have two homologous X chromosomes.

While Mendel could easily get multiple pea types from seed sources, Morgan was most likely the first person to desire diverse fruit fly species.

He was confronted with the laborious process of doing several matings and then microscopically analyzing huge numbers of progeny in quest of naturally occurring mutant individuals.

After many months of this, he grumbled, "Two years of labor squandered." I've been raising those flies for a long time and I've gotten nothing in return.”

Morgan persisted and was rewarded with the finding of a solitary male fly with white eyes rather than the typical red. The wild type is phenotypic for a characteristic that is most often found in natural populations, such as red eyes in Drosophila.

Traits that differ from the wild type, such as white eyes in Drosophila, are referred to as mutant phenotypes because they are caused by alleles that are thought to have started as alterations, or mutations, in the wild-type allele. Morgan and his students developed a notation for representing alleles in Drosophila that is still extensively used in fruit flies today.

In flies, the gene for a specific character gets its symbol from the first mutant (non–wild type) identified. Thus, in Drosophila, the allele for white eyes is represented by w. A superscript + designates the wild-type allele: w+ for the allele for red eyes, for example.

A number of gene notation systems for various species have been created throughout the years. Human genes, for example, are generally written in full capital letters, such as HTT for the gene implicated in Huntington's disease. (Some alleles, like this one, may utilize more than one letter.)

• The term sex-linked gene refers to A gene located on either sex chromosome. The human X chromosome contains approximately 1,100 genes, which are called X-linked genes, while genes located on the Y chromosome are called Y-linked genes. On the human Y chromosome, researchers have identified 78 genes that code for about 25 proteins (some genes are duplicates)

About half of these genes are solely expressed in the testis, and several are essential for adequate testicular function and sperm generation. The Y chromosome is almost entirely handed down from father to son. Because there are so few Y-linked genes, relatively few diseases on the Y chromosome are passed down from father to son.

WNT4 (on chromosome 1, an autosome) encodes a protein that stimulates ovarian development and is required for the development of female gonads.

An XY embryo with additional copies of the WNT4 gene can develop rudimentary female gonads. Overall, the interactions of a network of gene products like these determine sex.

The biochemical, physiological, and morphological characteristics associated with “males” and “females” are proving to be more complex than previously assumed, with several genes involved in their development. Because of the complexities of this procedure, there are several variants.

Some people are born with intermediate sexual (“intersex”) traits, or with physical features that do not match their sense of gender (“transgender” people). Sex determination is an active field of study that will offer more nuanced knowledge in the coming years.

Because men and females inherit a variable number of X chromosomes, their inheritance patterns differ from those created by genes placed on autosomes. A variety of human X-linked diseases, such as Duchenne muscular dystrophy, which affects roughly one out of every 3,500 men born in the United States, are far more dangerous than color blindness. The illness is marked by increasing muscular weakness.