sex linkage

lecture 16

What determines the sex of an individual?

• Sex chromosomes determine the sex of an individual

• Humans have 22 pairs of autosomes and 1 pair of sex chromosomes in their karyotype

Which sex chromosome has the primary sex determining genes?

• The Y chromosome has sex determining genes, not the X chromosome.

• The SRY gene (sex determining region of Y) on the Y chromosome is important for triggering testis development

What happens if the SRY gene is absent?

If the SRY gene is absent, ovaries will develop

Describe the characteristics of the Y chromosome.

The Y chromosome is Acrocentric. It has fewer genes, all of which relate to maleness1

Describe the characteristics of the X chromosome.

• The X chromosome is sub-metacentric

• . It has over 1600 genes, many of which govern traits not related to sex determination

Who determines the sex of the offspring in humans?

• The father determines the sex of the offspring

• An XY father can pass on either an X or a Y chromosome, resulting in a 50% chance of a female (XX) and a 50% chance of a male (XY) when combined with the mother's X chromosome

What is a sex-linked gene?

A sex-linked gene is a gene that is carried on a sex chromosome but is not involved in sex determination

Where are most sex-linked genes found?

• Most sex-linked genes are found on the X chromosome, called X-linked genes. There are not many sex-linked genes on the Y chromosome

Name some examples of traits governed by sex-linked genes.

• genes for blood clotting proteins, colour blindness, and muscle proteins. Specific conditions mentioned are haemophilia and Duchenne muscular dystrophy

How are alleles represented in crosses involving X-linked genes?

• The X and Y chromosomes are used, and the alleles are inserted as superscripts on the X chromosome.

• There is generally no corresponding allele on the Y chromosome, which leads to unusual patterns of inheritance

What type of inheritance pattern does Red-green Colour blindness follow?

It follows X-linked recessive inheritance

Are linked genes the same as sex-linked genes?

no

What are linked genes?

genes that are located close together on the same chromosome Because they are close together, they tend to be inherited as a single unit

How do linked genes affect the inheritance patterns compared to unlinked genes?

• Unlinked genes (on different chromosomes or far apart on the same chromosome) assort independently during meiosis, typically resulting in a 9:3:3:1 ratio in the F2 generation of a dihybrid cross..

• If genes are linked (close together on the same chromosome), they are inherited together, and there is no independent assortment.

• This typically results in a 3:1 ratio in the F2 generation of a dihybrid cross involving parental phenotype

Describe unlinked genes.

located on different chromosomes or are far apart on the same chromosome. They follow the principle of independent assortment