Principles of Genetics 1

Deviation from Mendel’s law

Sometimes during the crosses the ratio obtained does not obey Mendelian’s law.

Deviation from Mendel’s

law I:

1. Dominance:

a. Co-dominance b. Incomplete dominance

2. Lethal alleles

c. Dominance lethal alleles d. Recessive lethal alleles

3. Mutation:

a. Multiple alleles

4. Sex :

a. Sex- limited b. Sex- influences c. Sex- linkage

Main topics:

Deviation of Mutation

a. ABO type

b. Fur color of rabbits

Deviation of Sex

a. Sex limited

b. Sex influence

c. Sex linkage

Lab. 9

Deviation of Mendel’s first law “Monohybrid” part 2

Principles of Genetics 2

3. Multiple alleles:

In Mendel's studies, he proposed that there are two alleles for every gene, the dominant

of the two having its phenotype expressed in a heterozygote. However, a gene can

have more than two allelic forms segregating within a population.

These genes are referred to as having multiple alleles. This does not mean that the

gene in a particular individual possesses more than two alleles. An individual can only

have a maximum of two of the alleles, one maternal and one paternal, no matter how

many alleles exists in the population.

Examples:

ABO blood types are an example of multiple alleles.

Fur color in rabbits is determined by one gene with four different possible alleles.

1. ABO blood types

For the ABO gene, the three alleles are the IA, IB and i alleles. We

typically call these alleles "A," "B," and "O," but of course our rules for

assigning symbols to alleles demand that all three be represented by

some version of the same symbol. In this case, that common symbol

is the letter "I," which stands for “Isoagglutinogen."

By now, the concept of dominance should be familiar to you. Of course, things get a bit

more complicated when there are three alleles instead of just two. As the symbols

above should suggest, the i allele (the "O" allele) is recessive to both the IA and IB

alleles (the "A" and "B" alleles). The IA and IB show Co-Dominance. This means that in

an individual who is heterozygous for these two alleles, the phenotypes of both alleles

are completely expressed, thus producing blood type AB.

Principles of Genetics 3

Thus we have the following:

Genotype Phenotype

ii Type O

IAIA or IAi Type A

IBIB or IBi Type B

IAIB Type AB

ABO blood group antigens present on red blood cells and IgM

antibodies present in the serum:

Principles of Genetics 4

Genetic Analysis of Blood group:

P:

Gp:

F1:

2. Fur color in rabbits is determined by one gene with four different

possible alleles.

Fur color in rabbits is determined by a

single gene loci for which there are four

alleles. Four phenotypes are possible:

Black, Chinchilla (gray color caused by

white hairs with black tips), Himalayan

(white with black patches on extremities of

rabbit), and white.

The black allele (C) is dominant over all

other alleles, Chinchilla (Cch) is dominant

over Himalayan (Ch), and white or

Albino (c) is recessive to all others.

Figure: Fur color in rabbits

Principles of Genetics 5

Ex. Fur color in rabbits.

What is the genotype and the Phenotype of the parents and

offspring in hybrids following:

Wild x wild --------------- ¾ wild : ¼ Himalayan

Genetic Analysis: P:

Gp:

F1:

Wild x Himalayan ----------- ½ wild : ¼ Himalayan : ¼

Albino

Genetic Analysis: P:

Gp:

F1:

Gender & inheritance:

For most inherited traits, the gender of the bearer of the

genes is immaterial. Characteristics like free earlobes, fur

color, etc., generally operate the same in males as they do

in females. But there are exceptions. These fall into three

primary categories.

Sex limited traits

There are many examples where the action of a gene is influenced by the sex of the

individual. This is known as sex-limited inheritance (also known as sex-controlled

inheritance) and, in Butterflies, typically affects color, wing shape, markings and

behavior. The resulting sexual dimorphism results from the action of sex-limited

genes which, although present in both sexes, only have an effect in one sex. We have

two examples in the British Isles - the valesina form of the female Silver-washed

Principles of Genetics 6

Fritillary and the helices form of the female Clouded Yellow, where the named form is

only expressed in the female. In both cases, the allele controlling the form is dominant

(which has been proven through captive rearing and examining the ratios of offspring)

rather than recessive (which is what we might expect given the ratios we see in the wild,

which we'll come back to).

Now let's consider a couple of pairings. The figure below shows a pairing of a normal

male (that doesn't carry the valesina allele) with a valesina female. In this example,

we're considering both the sex chromosomes (X and Y), as well as the allele for wing

color (represented using the letter V). Since the male is XXvv, the only combination of

alleles in the gametes is Xv (as shown). We can see that we get equal numbers of

normal and valesina females in the offspring.

Figure: Sex limited

Principles of Genetics 7

Sex influenced traits

Sex influenced traits are also autosomal,

meaning that their genes are not carried on the sex

chromosomes. Again, what makes these traits

unusual is the way they are expressed

phenotypically. In this case, the difference is in the

ways the two genders express the genes.

One classic example of a sex influenced trait is

Pattern Baldness in humans (sometimes called

“male pattern baldness,” though the condition isn’t

restricted to males). This gene has two alleles,

“bald” and “non-bald.” The behaviors of the

products of these genes are highly influenced by

the hormones in the individual, particularly by the

hormone testosterone. In the presence of high levels of testosterone, the baldness allele

has a very powerful influence. In the presence of low levels of testosterone, this allele is

quite ineffectual. All humans have testosterone, but males have much higher levels of

this hormone than females do. The result is that in males, the baldness allele behaves

like a dominant allele, while in females it behaves like a recessive allele. As in all cases,

dominance only matters in the heterozygote, so this means that heterozygous males will

experience hair loss and heterozygous females will not. Even homozygous females may

experience no more than a thinning of their hair, but many develop bald spots or have

receding hairlines.

Ex. Baldness in human

Genetics

Analysis:

P: HH x hh

(♀ hair, ♂ baldness) x (♀ baldness, ♂ hair)

F1: Hh (♀ hair less, ♂ baldness)

F2: HH : 2 Hh: hh

Figure: Sex influenced

Principles of Genetics 8

Sex Linked Traits:

Sex Linked Traits are traits whose loci are literally on the sex chromosomes, so their

transmission from generation to generation is affected by the sex chromosome

complement of the individual. In any species with non-homologous sex chromosomes,

these traits can be significant.

The first demonstration of sex linkage was the white eye gene in Drosophila, the fruit fly

which has become so important to the study of classical genetics. Normal fruit fly eye

color is a dull brick red. Mutations in this gene cause the eyes to be white. The white

allele is recessive, but it was quickly determined that the inheritance pattern for this

gene was different from those of other genes being studied. In some kinds of mating,

reciprocal crosses produced different results, something which had never been

observed to happen with other genes. Not only that, but in some kinds of mating, the

results for the male offspring would be different from the results for the female offspring.

For instance, if a white-eyed female was mated to a red-eyed male, all of the female

offspring would be red eyed, but all of the male offspring would be white eyed.

It turned out that this particular eye color gene was literally located on the X

chromosome. Since females have two X chromosomes and males have only one,

genetic effects in the two genders are different. And since females contribute an X to

each offspring, male or female, and males contribute X chromosomes only to female

offspring, naturally transmission patterns were different in different kinds of mating.

“Linkage” refers to the physical connection that exists between genes whose loci are on

the same chromosome. Genes on the X chromosome are all linked to each other-thus

they are X-linked.

Principles of Genetics 9

Figure: sex linked in Drosophila melanogaster.

Ex. Sex linked in Drosophila melanogaster (♀ wild, ♂ white)

Genetics

Analysis:

P:

F1:

F2:

Ex. Sex linked in Drosophila melanogaster (♀ white, ♂ wild)

P:

F1:

F2:

Principles of Genetics 10

Gender Human Roach Butterfly Chicken

Male XY XO XX XX

Female XX XX XY XO

References:

http://biotech.gsu.edu/houghton/2107/lecture11.html

Yazer M, Olsson M, Palcic M; Olsson; Palcic (2006). "The cis-AB blood group phenotype: fundamental lessons in glycobiology". Transfus Med

Rev 20 (3): 207–17.doi:10.1016/j.tmrv.2006.03.002. PMID 16787828.

Muramatsu M, Gonzalez HD, Cacciola R, Aikawa A, Yaqoob MM, Puliatti C (2014)."ABO incompatible renal transplants: Good or bad?". WORLD

JOURNAL OF TRANSPLANTATION 4 (1): 18–29. doi:10.5500/wjt.v4.i1.18. PMID 11532188.

http://staff.jccc.net/pdecell/bio205/webct/medelproblems.html http://lhs.dcsdschools.org/UserFiles/Servers/Server_3858408/File/Shaw/sex-linked%20traits.pdf http://www.okstate.edu/artsci/zoology/ravdb/files/4.pdf

Principles of Genetics 11

1. Choose the right answer from the following:

A child is born with the blood type B. The mother of the child is blood type

O. Which of the following statements is most correct?

(A) The father must be type B

(B) The father could be type A

(C) The father passed along a recessive allele

(D) The mother could have passed along a type B allele

(E) The mother could have passed along a dominant allele

Hemophilia is a sex-linked recessive trait. A male hemophiliac and

phenotypically normal female have a girl that is a hemophiliac. All of the

following statements are correct EXCEPT:

(A) The daughter inherited a recessive gene from each parent.

(B) The daughter inherited a dominant allele from her mother.

(C) The mother is a carrier of hemophilia

(D) The genotype of the mother is X H X h

(E) The genotype of the father is X h Y

Suppose a newborn baby was accidentally mixed up in the hospital. In an

effort to determine the parents of the baby, the blood types of the baby and

two sets of parents were determined. Baby 1 had type O Mrs. Brown had type

B Mr. Brown had type AB. Mrs. Smith had type B Mr. Smith had type B.

a. Draw Punnett squares for each couple (you may need to do more than 1

square/couple).

b. To which parents does baby #1 belong? Why? Hint you may want to refer to your Punnett

squares.

Exercise 9

Principles of Genetics 12

A man heterozygous for blood type B marries a woman heterozygous for

blood type A. What is the chance that their first offspring will have type O?

Hemophilia in humans is inherited as an X linked recessive trait. A woman

whose father is hemophilia marries a man with normal clotting ability. What is

the probability that her first child will have hemophilia? Assume that the

woman's mother is homozygous dominant.

Red/Green color blindness is a sex linked traits. If a man with normal color

vision marries a colorblind woman, will all of the offspring be colorblind? If

not, who won't be colorblind (sons or daughter)?

Principles of Genetics 13

What is the probability of child will be bald if his father is homozygous and

balding, and his mother is homozygous and not balding?

A male homozygous for clubfoot reproduces with a normal homozygous

female. What are the genotypes and phenotypes of their children if

testosterone alters the phenotypic expression of the trait in the

heterozygous expression?

1. Let F represent normal feet

2. Let f represent clubfoot.

Two heterozygous individuals get married and have lots of children. The

father suffers with a painful condition called gout. What are the parent’s

genotypes? What are the genotypes and phenotypes of the children?

Let G represent no gout, and g represent gout.