Download Genetics

Punnett Squares and Probability
Punnett squares are a method in which all the possible offspring types are determined based on the
parents’ genes. The genes of individuals (represented by alleles) are called genotypes. The physical
appearance or phenotype of an individual is a result of what the genotype determines. For example, if
freckles are dominant over no freckles, the genotype Ff would have the phenotype of having freckles.
The parents’ genotypes determine what possible alleles are given to the offspring. The allele type varies
and according to the laws of segregation and independent assortment, two different genes with different
alleles separate completely and recombine in four possible gamete combinations. An easy way for
students to remember how to find the possible gametes, the acronym FOIL (first, outside, inside, last) is
often used. For example, if the parent genotype is AaBb, the four possible gametes are AB, Ab, aB, and ab. Using a Punnett
square, the gametes are combined in such a way as to determine all the possible genotypes. A ratio of the number of genotypes is
gathered by adding up all the same genotypes and comparing them to the others using a colon between the numbers. A ratio of the
phenotypes of the offspring are gathered in a similar manner.
1.
If freckles are dominant over plain cheeks, and cleft chin is dominant over a smooth chin, what would the genotype of a parent be who
is heterozygous freckled and heterozygous cleft?
2.
What are the possible gametes of the father? Use the FOIL method to determine.
3.
Using a Punnett square, what are the possible offspring of the parents if they both are heterozygous for freckles and cleft chin?
4.
What are the genotypic ratios and phenotypic ratios of the offspring of those two parents?
F. Sex-Linked Traits
Sometimes a particular trait is found on a sex chromosome, usually X. These genes are
called sex-linked genes only because they are located on the sex chromosome X. The
characteristic has nothing to do with the sex of the individual. Since females have two X
(XX) and males only have one (XY), males have a higher chance of expressing a defective
recessive gene since they don’t have another X to act as the dominant X. Females with only
one defective allele are said to be carriers. Their phenotype is normal and they do not
express the disorder. A Punnett square to determine sex-linked inheritance must include the
sex chromosomes X and Y using a lowercase superscript to denote the defective recessive
gene located on the X chromosome. A few sex-linked disorders are commonly found worldwide. The first is colorblindness (noted
as Xc) in which an afflicted individual inherits a defective gene coding for the color-detecting cones of the eye’s retina. This
individual may have a hard time distinguishing two colors. A second type of sex-linked disorder is the blood clotting defect called
hemophilia. An individual with hemophilia cannot produce adequate blood clots and may bleed to death as a result. This disease is
noted as Xh where the h is the defective blood-clotting protein. A third type of sex-linked disease is Fragile X syndrome. A person
with Fragile X inherits an addition of 600+ nucleotides on the X chromosome which results in abnormal facial features and
intellectual disabilities. This is denoted as Xf. The fourth and final common sex-linked disorder is Duchenne’s muscular dystrophy
(Xd) in which the individual inherits a defective muscle protein causing progressively weakened muscles. The average life span for
someone with Duchenne MD is 25 years.
5.
What is a sex-linked trait?
6.
Why are males more prone to inherit the disease or disorder?
7.
Why are females considered carriers? Why can’t males be carriers?
1
8.
Cross a male afflicted with colorblindness and a normal woman.
9.
If a female carrier of Fragile X syndrome has children with a normal male, what are the chances that a boy will be born with Fragile X
syndrome?
10. Cross a male hemophilia with a female carrier of hemophilia. What are the chances they will have a girl with hemophilia?
11. Cross a female carrier of Duchenne’s muscular dystrophy with a healthy male. What are the chances the will have a girl with
Duchenne?
I. Incomplete Dominance
hh
H’H’
H’h
In simple heredity, an uppercase allele means it is a dominant allele and its phenotype is always
expressed. This is not always the case as some “dominant” alleles aren’t truly dominant. These are
called incompletely dominant where the heterozygote shows a blend of the incomplete dominant
and the recessive. These uppercase alleles may be designated with a ‘ or “prime” to signal that they
do not act dominantly. In the case of flowers, snapdragons’ red pigment behaves incompletely
dominant where R’r is pink! In order to see the “dominant” phenotype, the genotype must be
homozygous dominant. The same holds true for the recessive phenotype, as is typical. In humans,
nose size and hair texture act similarly. A large nose (L) is incompletely dominant (L’) over a small
nose (l) making the heterozygote (L’l) a medium-sized nose. Curly hair is incompletely dominant (C’)
over straight hair (c) making the heterozygote individual wavy-haired (C’c).
12. Why is using the character ‘ a good practice when noting incomplete dominance?
13. Blue hair in aliens is incompletely dominant over red hair. Cross two heterozygous aliens.
a. What are the genotypes?
b.
What phenotype is the heterozygote?
14. Wide-set eyes are incompletely dominant over close-set eyes.
a. What are the gentoypes?
b.
What phenotype is the heterozygote?
2
Multiple Alleles
So far, all genes have been coded for by two alleles, one from the father and one from the mother. This will always be the case as
each parent can only donate one allele. Certain genes are coded for by more than two alleles,
so the phenotype depends on which alleles are passed down and the order of dominance.
This inheritance pattern is termed multiple alleles and although it uses more than two alleles
for one gene, each allele uses one letter, often times with superscripts to differentiate
variances. A prime example of this is in human blood types where the letter I denotes the
protein immunoglobulin. A genotype of IA denotes the phenotype type A blood, IB denotes
type B blood, and the recessive i denotes the absence of A or B which is termed type O
blood.
15. How many different letters of genes are used to denote multiple alleles? Why?
16. In corn kernel color, yellow is dominant over white, white is dominant over blue, and blue is
dominant over red. What should the alleles be to represent each phenotype?
17. Cross a pure bred blue corn plant with a heterozygous yellow-white corn plant.
K. Codominance
The final complex pattern of heredity is termed codominance where two dominantly inherited alleles code for the same trait. Since
both alleles are dominant, the phenotype shows BOTH phenotypes, not a blend, but each phenotype equally. In the case of
humans, two particular traits use codominance. The first trait is sickle-cell anemia where the heterozygous individual has both
round red blood cells (R) and sickle-shaped red blood cells (S). The genotype for this may be RS. Another human trait that uses
codominance is blood type where type A and type B are dominant, therefore an individual with the IA and IB allele is said to be
blood type AB. This type of inheritance pattern is also frequently found in flower petals, and certain furry mammals such as roan
cattle, a codominant red and white phenotype (RW).
18. Sickle-cell anemia is a codominant trait. Cross two heterozygous individuals.
19. What percent of offspring may contract sickle-cell?
20. Why is blood type in humans considered both multiple alleles and codominance?
3