Genetics - Sex Linked Traits

GENETICS Sex-Linked Traits Robert Perry MALIBU HIGH SCHOOL

3.

SEX INHERITANCE & SEX-LINKED TRAITS

CHROMOSOMES

ALL HUMAN CELLS (except gametes) HAVE 23 PAIRS of CHROMOSOMES

THERE ARE 22 PAIRS of SOMATIC CHROMOSOMES

and 1 PAIR of SEX CHROMOSOMES

XX sex chromosomes

XY

Let’s use a Punnett Square to predict the offspring.

XY XX

First, write the parents chromosomes

XY XX

X X X Mother’s ova.

Y

Father’s sperm.

Second, write the possible gamete types.

XY XX

X Y X XX XY X XX XY

Next, let’s match up the gametes to predict offspring

XY XX

X X X

Y Offspring: 50% female 50% male

XX sex chromosomes carry the genes for gender and secondary sexual characteristics.

XY

The x-chromosome is much larger than “y� and thus carries genes for traits other than sex and secondary sexual characteristics.

color blindness

The gene for color blindness is carried on the x-chromosome.

X

c

symbol

Sample x-linked problem:

X X c

NORMAL, BUT “CARRIER” FEMALE Breeds with

XY

NORMAL MALE

XX c XX XX c X Y XY c

c

COLOR BLIND FEMALE NORMAL, BUT “CARRIER” FEMALE NORMAL FEMALE

COLOR BLIND MALE NORMAL MALE

normal father

XX c

carrier mother

XY

normal father

X X Xc X c

carrier mother

XY X

Y

normal father

XY

X Y c X X Xc XcX XcY X XX XY carrier mother

normal father

XY

X Y c X X Xc XcX XcY X XX XY carrier mother

Note: There are 3 ways to ask TEST Q’s: 1-all offspring 2- male offspring 3-female offspring

normal father

X c X X Xc XcX X carrier mother

XY Y Offspring: 1/4 carrier female

normal father

X c X X Xc XcX X XX carrier mother

XY Y Offspring: 1/4 carrier female 1/4 normal female

normal father

XY

X Y c Offspring: X X Xc XcX XcY 1/4 carrier female 1/4 normal female 1/4 color blind male X XX carrier mother

normal father

XY

X Y c Offspring: X X Xc XcX XcY 1/4 carrier female 1/4 normal female 1/4 color blind male X XX XY 1/4 normal male carrier mother

normal father

XY

X Y c X X Xc XcX X XX XY carrier mother

Total: 3/4 NOT color blind

normal father

XY

X Y c Total: X X Xc XcX XcY 3/4 NOT color blind X XX XY 1/4 ARE color blind carrier mother

normal father

X c X X Xc XcX X XX carrier mother

XY Y Total: 0/2 females color blind

normal father

XY

X Y c X X Xc XcX XcY X XX XY carrier mother

Total: 0/2 females color blind 1/2 males color blind

REVIEW CHECKPOINT:

REVIEW CHECKPOINT: 1. What are the chromosomes for a male? female?

REVIEW CHECKPOINT: 1. What are the chromosomes for a male? female?

2. What percent of all offspring will be male? female? Why ?

REVIEW CHECKPOINT: 1. What are the chromosomes for a male? female?

2. What percent of all offspring will be male? female? Why ?

3. What is an x-linked trait ? Why aren’t there many y-linked traits ?

REVIEW CHECKPOINT: 1. What are the chromosomes for a male? female?

2. What percent of all offspring will be male? female? Why ?

3. What is an x-linked trait ? Why aren’t there many y-linked traits ?

4. Give at least one example of an x-linked trait.

REVIEW CHECKPOINT: 1. What are the chromosomes for a male? female?

2. What percent of all offspring will be male? female? Why ?

3. What is an x-linked trait ? Why aren’t there many y-linked traits ?

4. Give at least one example of an x-linked trait.

5. If a color blind father and a completely normal (non-carrier) mother have 14 kids, give the expected %’s.