1. Sex-Linked: Following the X & Y Chromosomes
Section 5.3

2. Sex-Linked Following the X & Y Chromosomes
Learning Goal: To learn about the inheritance of sex-linked traits.
Success Criteria:
I know I am succeeding when I can…
define autosomal inheritance and sex-linked inheritance
explain why females are usually carriers of X-linked traits while males are usually affected by X-linked traits
calculate the probability of offspring expressing or being a carrier of a sex-linked trait
determine the mode of inheritance of a trait from a pedigree

3. What Do You See? Color blindness is an x-linked disorder
Take a color blindness test!

4. Sex-Linked Recall… humans have 23 pairs of chromosomes
one pair of chromosomes consists of the sex chromosomes
the other 22 pairs are autosomes, the non-sex chromosomes
Different organisms have different numbers of chromosomes.
Humans have 23 pairs of chromosomes.
One set of chromosomes consists of the sex chromosomes, while the other 22 sets are autosomes, the non-sex chromosomes.

5. Sex-Linked Cont’d…
autosomal inheritance: inheritance of alleles located on autosomal (non-sex)chromosomes
traits inherited on autosomal chromosomes affect males and females equally
With autosomal inheritance, both males and females are affected equally, since there is no difference between the autosomes of males and the autosomes of females.

6. Sex-Linked Cont’d…
some alleles that cause genetic disorders are found on the sex chromosomes (mostly the X chromosome)
Sex-linked: describes an allele that is found on one of the sex chromosomes, X or Y, and when passed on to offspring is expressed
X-Linked: an allele with the disorder is found on the X chromosome
more males than females will express the recessive allele but more females are carriers of the recessive X-linked allele
Y-linked disorders also exist and are passed on from father to son.
One reason there are fewer Y-linked disorders than X-linked ones is that the Y chromosome is small and does not carry as much genetic information as the X-chromosome.
Reduced fertility
in males can be caused by a Y-linked disorder. Males who possess this disorder can have children using medical intervention.

7. Sex-Linked Cont’d…
females (XX) have two copies of the gene, but males (XY), with only one X chromosome, have only a single copy
With autosomal inheritance, both males and females are affected equally, since there is no difference
between the autosomes of males and the autosomes of females.

8. Sex-Linked Cont’d…
if a male inherits the X chromosome from a mother who carries the recessive allele, he will express the disorder because the Y chromosome cannot mask the effects of that allele
a female must inherit two copies of the recessive gene—one on each X chromosome—in order to express the disorder
With autosomal inheritance, both males and females are affected equally, since there is no difference
between the autosomes of males and the autosomes of females.

9. Sex-Linked Cont’d…
the male cannot inherit an X-linked disorder from his father, since a father passes on a Y chromosome to a son
examples of X-linked inheritance are red–green colour blindness, hemophilia A, and male-pattern baldness
With autosomal inheritance, both males and females are affected equally, since there is no difference
between the autosomes of males and the autosomes of females.

10. Practice Problems
Colour blindness is an X-linked disorder and the allele for colour blindness is recessive to the normal allele for colour vision. What are the possible genotypes for females and males?

11. Practice Problems Cont’d…
A male normal for color vision mates with a woman who is a carrier of the color blind allele.
What is the probability of having a son who is color blind?
What is the probability of producing a daughter who is a carrier of the color blind allele?

12. Sex-Linked Following the X & Y Chromosomes
Learning Goal: To learn about the inheritance of sex-linked traits.
Success Criteria:
I know I am succeeding when I can…
define autosomal inheritance and sex-linked inheritance
explain why females are usually carriers of X-linked traits while males are usually affected by X-linked traits
calculate the probability of offspring expressing or being a carrier of a sex-linked trait
determine the mode of inheritance of a trait from a pedigree

13. Homework
5.3 questions #1-5