Download 1. Single gene traits

Homework: Lab 8
ANSWER KEY
1. Single gene traits:
Define:
phenotype – the outward or physical expression of the genetic code of an organism
genotype – the genetic code of an organism; which alleles are present
dominant – the allele that determines the phenotype of a heterozygote and masks the expression of
the recessive allele
recessive – the allele that is masked by the presence of a dominant allele in a heterozygote. The
recessive phenotype is only expressed in homozygotes.
Homozygous – having two copies of the same allele of a gene
Heterozygous – have one copy of each allele of a gene
What is the law of segregation? Each gamete receives only one of each parent’s genes for each trait
What is the law of independent assortment? Alleles for one trait are distributed to gametes independently of
the alleles for another trait.
In tomatoes, fruit color is controlled by a single gene. The red fruit allele is dominant over the yellow fruit
allele. Suppose a gardener is trying to develop a new variety of tomatoes. She has one nice heterozygous
red-fruited tomato plant that was the result of last year’s cross between a homozygous red-fruited and a
yellow-fruited tomato. This year she wants to cross it with another yellow-fruited tomato. Draw a Punnet
square that predicts the outcome of this year’s cross. Use R for the dominant allele and r for the recessive
allele. What proportion of tomato plants will be red fruited and what proportion will be yellow fruited?.
R
Rr
Rr
r
r
r
rr
rr
2/4 or 50% are red-fruited (Rr) and 2/4 are 50% are yellow-fruited (rr)
For another type of tomato color is controlled by TWO genes. One gene, the Pale gene, determines
whether the fruit has color. The dominant (P) allele prevents other color alleles from being expressed, so
the fruit turns pale. The recessive form (p) allows other colors to be expressed. A second gene causes the
fruit to be either yellow (Y) or red (y). So for these tomatoes:
• Inheriting one P allele makes the fruit pale regardless of any other alleles inherited.
• If the tomato gets two p alleles and at least one Y allele, the fruit will be yellow.
• If the tomato gets the genotype ppyy, it will be red.
The gardener crosses two tomatoes that she knows have the genotype PpYy. Make a dihybrid Punnet
square to predict the outcome of this cross. What is proportion of each phenotype and how is it different
from the phenotypic ratio in the first type of tomato fruit above?
PY
Py
pY
py
PY
PPYY
PPYy
PpYY
PpYy
Py
PPYy
PPyy
PpYy
Ppyy
pY
PpYY
PpYy
ppYY
ppYy
py
PpYy
Ppyy
ppYy
ppyy
1/16 or 6.25% of fruits are red
3/16 or 18.75% of fruits are yellow
12/16 or 75% of fruits are pale
In this example fruit color is controlled by TWO genes, but they are independently assorted, so an
individual must be recessive for both genes to have a red phenotype, must be recessive for the pale gene
and homozygous dominant or heterozygous for the color gene to be yellow, and if it is homozygous
dominant or heterozygous for the pale gene, it will be pale. Thus there are three phenotypic possibilities,
but 16 possible allelic combinations
Homework: Lab 8
ANSWER KEY
3. X-linked traits
Baldness is an X-linked trait in humans. Men who inherit an X chromosome with the baldness allele will
lose their hair.
This is Angus. Angus’s mom has great hair. Angus’s dad also has great hair.
Angus’s Uncle Jack (his mom’s brother) has great hair. Angus’s maternal
grandfather went bald at age 18. What is the probability that Angus will go
bald? To help you with this problem, first write the genotype of Angus’s mom
and dad, and then make a punnet square.
Mom’s genotype XBXb – we know this because although her dad is bald, her
brother is not, thus mom has one good hair gene and one bald gene.
Dad’s genotype XBY – Dad has good hair, so his one X chromosome must
contain the good hair gene.
XB
Y
XB
X BX B
X BY
Xb
X BX b
X bY
Since Angus is a boy baby, we want to look at the babies that have on X and one
Y. Angus thus has a 50% chance of inheriting the Bald gene
b. If Angus grows up to marry Brynn- a beautiful girl with a bald dad- what is the minimum probability
that each of his sons will be bald?
Brynn’s dad is bald, thus Brynn must have at least one copy of the bald gene on her X chromosomes.
This mean’s that Angus’s sons each have at least have a 50% chance of being bald.
4. Single genes with multiple alleles: Blood typing
Angus’s mom has blood type A and so does his dad. Angus’s blood type is O. How is this possible?
(Note: Angus’s mom is a totally honorable person who absolutely did not engage in infidelity)
Blood type is coded by multiple genes. A and B are codominant and O is recessive. If Angus’s mom and
Dad each have the genotype AO, then Angus has a ¼ or 25% chance of being OO and thus having blood
type O.
A
O
A
AA
AO
O
AO
OO
If Angus gets a new baby brother, what is the probability that he will have blood type A? B? AB? O?
Every offspring of two parents represents the same probability so we can just use the same punnet square
we used in question 4. Angus’ baby brother has a ¼ or 25% chance of being blood type O, and a ¾ or
75% chance of being A. There are no B genes in the family, so he has zero probability of being type AB
or B.