A valgus angle is that remarkable structure in the human knee that allows for bipedal locomotion by placing our upper body weight and our center of gravity right over our feet. Besides this unique trait, what else defines us as "human"? This blog is part of a course in Physical Anthropology through the College of the Canyons, designed to help my students explore this question.

Mendelian Genetics Answers




















 
Mendelian Genetics Assignment 
Here are the answers to the problem set for this week on Mendelian Genetics.  I’ve provided explanations for each answer, so read through them carefully to understand how to solve problems such as these.
 

 1.   Predict the likely genotypic and phenotypic offspring ratios produced between two parent pea plants with regard to seed color (with yellow being dominant (Y) and green being recessive (y)) if one parent is heterozygous for the trait and the other is homozygous recessive.
Genotype: 
1 Yy: 1 yy (2:2 okay)
Phenotype: 
1 yellow : 1 green (2:2 okay)


Predict likely offspring from two parents – assume each combination equally likely
Homozygous Recessive Parent (yy - green)
y
y
Heterozygous Parent
(Yy - Yellow)
Y
Yy
(yellow)
Yy
(yellow)
y
yy
(green)
yy
(green)




2.   Suppose that you take two parent pea plants that are dominant the trait of seed color (yellow) and cross them.  The result is offspring that all produce yellow seeds.  Does this mean that both parents must have been homozygous dominant?  Why or why not?
No!  Just because the offspring are all dominant in their phenotypes does not mean their genotypes are homozygous dominant!  You could get this offspring result if one parent was homozygous dominant and the other is heterozygous.  Offspring will either be homozygous dominant or heterozygous.
Predict likely offspring from two parents – assume each combination equally likely
Homozygous Dominant Parent (YY - yellow)
Y
Y
Heterozygous Parent
(Yy - Yellow)
Y
YY
(yellow)
YY
(yellow)
y
Yy
(green)
Yy
(green)




3.  A woman takes a man to court claiming that he is the father of her son and that the man should be required to pay child support.  The man’s blood type is A, the woman’s is AB and her son’s blood type is B.  Is it possible for the man to be the boy’s father?  Explain your answer. 
The problem here is that while we know the mother’s blood type (AB) we don’t know the man's, which could be AA or AO.  Let’s run both of them through punnett squares to see if either is capable of producing a child with blood type B (BB or BO).
Predict likely offspring from two parents – assume each combination equally likely
Mother (AB)
A
B
Homozygous
Male
(AA)
A
AA
(A)
AB
(A)
A
AA
(A)
AB
(AB)
If the man is homozygous genotype AA, it isn’t possible that the man can be the father because it is impossible to produce a child with blood type B.

Predict likely offspring from two parents – assume each combination equally likely
Mother (AB)
A
B
Heterozygous Male
(AO)
A
AA
(A)
AB
(AB)
O
AO
(A)
BO
(B)
If the man is heterozygous (AO), then one out of four offspring (25%) are likely to be blood type B, with a heterozygous genotype (BO).


The man’s paternity cannot be ruled out based upon blood type inheritance patterns.  If the father is heterozygous for bloodtype A, he has a chance of producing a phenotypic B child 25% of the time.  To know for sure, we would have to know if the father is homozygous or heterozygous for bloodtype A.  Understand that this only means he *can* be the father.  To confirm actual paternity would require a DNA test.

4.   Color-blindness is a sex-linked trait, with the normal vision phenotype being dominant (C) and the color blind phenotype being recessive.  Answer the following questions, making sure to explain your answers completely.
      
      (a) If you are male and your father is colorblind, what is the chance (expressed as a percentage) that you will inherit his trait for colorblindness?
Predict likely offspring from two parents – assume each combination equally likely
Father (cY – colorblind)
c
Y
Homozygous
Mother
(CC)
C
Cc
(Carrier Female)
CY
(Not cb Male)
C
Cc
(Carrier Female
CY
(Not cb Male)
It is not possible for a son to inherit colorblindness from his father.  Fathers pass the colorblind allele to their daughters.




    (b)  What if your mother is a carrier for colorblindness (but your father has normal vision), what is the chance of you being colorblind?


Predict likely offspring from two parents – assume each combination equally likely
Father (CY – normal vision)
C
Y
Heterozygous
(Carrier) Mother
(Cc)
C
CC
(Not cb Female)
CY
(Not cb Male)
c
Cc
(Carrier Female)
cY
(cb Male)
If a mother is a carrier for colorblindness, there is a 50% chance that a son will inherit the colorblind allele and express the colorblind phenotype.


 Notice that it matters whether you specify a generic child or a specific sex of the child.  When you specify son, there are only two options if you are male, colorblind or normal vision.  There is therefore a 1 in 2 (or 50%) chance that you will inherit the colorblind allele from your mom and be colorblind.  But if you generalize the question and ask the chances of having a colorblind child, there are four options there and you have a 1 in 4 (or 25%) of being a colorblind male (as opposed to any of the other three options).


5.   Use the heredity chart to determine the answers to the following questions.  Make sure you answer the questions completely, explaining why you answered the way you did.  The trait of interest here is tongue-curling.  Males are indicated with a square, females are indicated by a circle.   The individuals in black are able to curl their tongues.   Individuals in white cannot curl their tongues.
·         What is the genotype of individual #1?   This individual produced a homozygous recessive offspring.  The only way this could have happened is if they are heterozygous themselves.
·         What is the phenotype of the individual #2?     This individual is dominant for the expression of tongue curling, the same as his father.
·         What is the genotype of individual #3?     Because all of his offspring express the trait for tongue curling, this suggests that this is a dominant trait and that individual #3 is homozygous dominant for the trait (CC)
·         Is tongue curling a dominant or a recessive trait?     This is actually the first question you needed to answer.  Because the offspring of individual #3 are all the same phenotype of their father, and none of their mother, it suggests a dominant trait pattern with tongue-curling (C) being dominant and non-curling (c) being recessive.
·         Is this a sex-linked trait?  Why or why not?    All offspring of individual # 3 received the curling trait regardless of their sex.  This is not indicative of a sex-linked trait.


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