BIO 110 Dr. Ely

Chapter 10 Notes – Patterns of Inheritance, Part 2

I. Non-Mendelian Genetics – Beyond Mendel’s Laws

a. For some traits, the expected probabilities associated with the simple dominant-recessive

system are not observed due to 1) polygenic inheritance, 2) incomplete dominance and 3)

codominance

b. Polygenic inheritance

i. Multiple genes can, and often do, influence a single trait. The result is an additive effect

of the various gene products, which creates a continuous distribution of traits.

ii. For example:

1. The height of a human being is not either 5 feet tall or 6 feet tall, but rather a

continuum of different heights are observed in the human population.

2. Skin color and eye color are highly variable from individual to individual.

c. Incomplete dominance

i. Some genes have alleles that are neither dominant nor recessive. Heterozygotes display

an intermediate phenotype, like a blend of the two alleles.

ii. An example is in four o’clock flowers, which have a gene for flower color with two

alleles: red and white. When the genotype is homozygous for either allele, that color is

expressed. However, when the genotype is heterozygous the flower color is neither red

nor white. Instead, it is pink – a blending of the two alleles.

d. Codominance

i. Other genes have alleles that are expressed whenever they are present, leading to some

heterozygotes that express both phenotypes at the same time.

ii. An example is human blood type.

1. Types A, B, and O refer to the presence of molecular markers called antigens on

the surface of red blood cells.

a. The IA allele codes for A antigens and is dominant

b. The IB allele codes for B antigens and is also dominant

c. The i allele codes for no antigens and is recessive

2. If the IA or IB alleles are paired with the recessive allele then the antigen will be

the dominant one.

3. However, an individual who inherits both IA and IB alleles will express both A and

B antigens on the surface of their red blood cells. These individuals have the AB

blood type.

4. Note that codominance results in the full expression of both phenotypes while

incomplete dominance results in a partial expression (a blend) of both

phenotypes

II. Sex-Linked Inheritance

a. The 23rd pair of chromosomes are our sex chromosomes. Females have two homologous X

chromosomes while males have two non-homologous chromosomes, X and Y.

b. The Y chromosome contains genes that give rise to male characteristics. These genes are not

present on the X chromosome. Thus, X and Y are two very different chromosomes which is why

BIO 110 Dr. Ely

they are non-homologous. However, they do find each other during meiosis and separate as do

the other pairs. This leads to some sperm cells possessing a Y chromosome (which would give

rise to a male offspring) and some sperm cells possessing a X chromosome (which would give

rise to a female offspring).

c. The X chromosome is much larger than the Y and has many more genes, most of which do not

have anything to do with gender.

d. Because males have only one X chromosome, males have only one allele for each gene on the

X. Females have two alleles for every X-linked gene because they have two X chromosomes.

e. This means that whichever allele is on a male’s single X chromosome, dominant or recessive, is

the one that will be expressed.

f. Example: Color-blindness (b) is a recessive X-linked trait. Color-vision (B) is dominant.

i. To keep track of the gender of the offspring we use X and Y with the alleles as a

superscript on the X chromosome only (because the Y does not have these genes).

ii. So,

1. a color vision male has the genotype XBY

2. a color-blind male has the genotype XbY

3. a color-blind female has the genotype XbXb

4. a color-vision female is either XBXB or XBXb

iii. If we were to cross a color-vision male with a female who is a carrier (i.e. heterozygous)

we would set it up like this:

iv. We would then complete the Punnett square as usual, keeping track of the X and Y

chromosomes so we know the gender of the offspring:

v. We could describe these results in a few different ways:

1. There is a 25% chance that these two parents will produce a color-blind boy (or a

color-blind child in general).

2. If the child is a boy, there is a 50% chance he will be color-blind.

3. There is a 50% chance that these two parents will produce a color-vision female.

4. These two individuals will never produce a color-blind female.