Not-so-Simple Inheritance

Patterns

Remember…

• Genes are sections of a chromosome that have coded instructions for making proteins.

• Alleles are different forms of a gene.

Simple Inheritance

• 2 alleles • 2 phenotypes• One allele is

dominant• The other allele is

recessive• The DOMINANT allele

completely masks the recessive allele

Mendel Got Lucky

• In many ways Gregor Mendel was quite lucky in discovering his 4 genetic laws. 

• He happened to use pea plants. 

• Things in nature aren’t always so simple.

Not-so-Simple Inheritance• 4 Main Types

– Codominance– Incomplete Dominance– Multiple Alleles– Polygenic Traits

Codominance• “Co” means together!• Codominance = two

alleles are dominant together.

• This produces a third heterozygous phenotype where BOTH traits are expressed TOGETHER

• Ex: Human Blood Type– A– B– O– AB

Red Cow x White Cow Roan CowRoan cattle have

both red and white hairs mixed in

together!

Codominance

red x white red & white spotted

8

Codominance• Two alleles are expressed

(multiple alleles) in heterozygous individuals.

• Example: blood type1. type A = IAIA or IAi2. type B = IBIB or IBi3. type AB = IAIB

4. type O = ii

copyright cmassengale

9

Codominance ProblemExample: homozygous male Type B (IBIB)

x heterozygous female Type A (IAi)

IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi

IB

IA i

IB

copyright cmassengale

10

Another Codominance Problem

• Example: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

copyright cmassengale

11

CodominanceQuestion:

If a boy has a blood type O and his sister has blood type AB,

what are the genotypes and phenotypes of their

parents?

boy - type O (ii) X girl - type AB (IAIB)

copyright cmassengale

12

Codominance•Answer:

IAIB

ii

Parents:genotypes = IAi and IBiphenotypes = A and B

IB

IA i

i

copyright cmassengale

Incomplete Dominance• Incomplete

Dominance = neither of the two alleles is completely dominant

• This produces a third heterozygous phenotype where the traits are BLENDED.

Amel & Ultra are incompletely dominant to each other.

Amel x Ultra Ultramel

Incomplete Dominance

red flower x white flower PINK flower

Multiple Alleles

• Any of a set of three or more alleles.

• Only two of alleles can be present in a diploid organism.

• Examples:– Human Blood Type – Human skin, hair, and eye

colors

Polygenic Traits• Result of the interaction of several genes• Human skin, hair, and eye color

– Influenced by more than one gene at different locations on the chromosomes

• Human Blood Pressure– Not just 1 “blood pressure” gene with several alleles– Caused by several genes that control other traits such

as weight, cholesterol level, kidney function, etc…

• Human Height– The combined size of all of the body parts from head

to foot determines the height of an individual. 

Role of Environment• Our environment also plays a key role in

how traits are expressed.• Genes can be turned on/off due to

external environmental factors.• Examples:

– Stress– Nutrition– Chemical exposure– Light– Temperature

Example of Environmental Influence

• Human Height is heavily influenced by hormones produced in the pituitary gland and our diet during childhood and puberty.

• Usually, about 10% of an individual's height is due to the environment.

Sample Questions

1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

Sample Questions

3. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritance does this illustrate? b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

Sample Questions

1. A cross between a blue blahblah bird & a white blahblah bird produces offspring that are silver.  The color of blahblah birds is determined by just two alleles. a) What are the genotypes of the parent blahblah birds in the original cross? b) What is/are the genotype(s) of the silver offspring? c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?

Sample Questions

2. The color of fruit for plant "X" is determined by two alleles.  When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit.  What are the genotypes of the parent orange-fruited plants?