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.