110 X Linkage

Summary

This video explains the concept of sex linkage, focusing on how genes on sex chromosomes (X and Y) affect inheritance patterns in mammals. It highlights the differences between the X and Y chromosomes, with the X chromosome containing more genes than the Y chromosome. In particular, the video explores how X-linked traits, such as red-green color blindness, are inherited in males and females. The inheritance patterns of these traits are demonstrated through Punnett squares, showing the likelihood of offspring inheriting dominant or recessive alleles for color blindness from their parents. The explanation emphasizes the hemizygous condition in males and carriers in females.

Takeaways

• 😀 Genes are considered linked if they are found on the same chromosome, affecting inheritance patterns.
• 😀 Sex linkage refers to genes found on the sex chromosomes (X and Y), which differ from autosomal genes.
• 😀 In mammals, sex chromosomes determine the sex of an individual: XX for females and XY for males.
• 😀 The X chromosome is much larger than the Y chromosome and contains over 1100 genes, while the Y chromosome contains only 78.
• 😀 Males have only one X chromosome, making them hemizygous for X-linked genes, unlike females who have two X chromosomes.
• 😀 X-linked genes in males can only be expressed as either dominant or recessive, since males have only one copy of the X chromosome.
• 😀 The term 'hemizygous' refers to males who have only one copy of an X-linked gene, either dominant or recessive.
• 😀 An example of X-linked inheritance is red-green color blindness, which is carried on the X chromosome.
• 😀 A colorblind man (X-linked recessive allele) marries a homozygous dominant woman, resulting in daughters who are carriers and sons who are unaffected.
• 😀 If a carrier daughter (heterozygous for color blindness) marries a man with normal color vision, their children have a 25% chance of being colorblind.
• 😀 In the offspring of a carrier woman and a normal male, there is a 25% chance of having a colorblind son, and a 75% chance of a child with normal vision.

Q & A

What does the term 'sex linkage' refer to?

Sex linkage refers to the inheritance of genes found on the sex chromosomes, typically the X and Y chromosomes in mammals, which affects the pattern of inheritance of certain traits.

What are the main differences between X and Y chromosomes?

The X chromosome is much larger than the Y chromosome and contains over 1,100 genes, while the Y chromosome has only around 78 genes. The X and Y chromosomes also differ in how they determine an individual's sex.

How does sex determination work in mammals?

In mammals, males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). The sex of the offspring is determined by whether an X-bearing sperm or a Y-bearing sperm fertilizes the egg.

What is the term used for males who have only one copy of a gene on the X chromosome?

Males with only one copy of a gene on the X chromosome are referred to as 'hemizygous.' They cannot be homozygous or heterozygous for X-linked traits.

How does gene inheritance differ for males and females in relation to X-linked traits?

Females, with two X chromosomes, can be homozygous or heterozygous for X-linked traits, while males, with only one X chromosome, are hemizygous and can only express the trait present on their X chromosome.

What is an example of a sex-linked trait, and how is it inherited?

Red-green color blindness is an example of a sex-linked trait, inherited through the X chromosome. Males with the recessive allele for color blindness will express the trait, while females need two copies of the recessive allele to express it.

If a homozygous dominant female for normal vision marries a colorblind male, what are the chances for their children to inherit color blindness?

If a homozygous dominant female (X^N X^N) marries a colorblind male (X^n Y), all daughters will be carriers (X^N X^n) and all sons will have normal color vision (X^N Y). No sons will inherit color blindness.

What are the possible genetic outcomes for children of a female carrier for color blindness and a normal male?

The offspring have a 25% chance of having a colorblind son (X^n Y), 25% chance of having a normal vision son (X^N Y), 25% chance of having a homozygous dominant daughter (X^N X^N), and 25% chance of having a heterozygous carrier daughter (X^N X^n).

Why are males more likely to express recessive X-linked traits than females?

Males are more likely to express recessive X-linked traits because they only have one X chromosome. If the recessive allele is present on that single X, they will express the trait. Females, with two X chromosomes, need two copies of the recessive allele to express the trait.

What is the significance of the Y chromosome in determining male sex?

The Y chromosome contains the SRY gene, which is crucial for determining male sex. It triggers the development of male characteristics during embryonic development.