Mendel’s Peas were ideal for Mendel’s Peas were ideal for learning about inheritance, but learning about inheritance, but they do not represent the norm… they do not represent the norm…

• Traits in pea plants are determined by just two alleles

• In peas, one allele is clearly dominant & the other is clearly recessive

• However, things aren’t always this clear-cut and simple in the world of genetics.

What if Mendel looked at mice?

If a female black mouse and a male white mouse were crossed, what will the offspring look like? - 100% are GREYGREY

If the F1 offspring were crossed, what will there offspring look like?- 25% black- 50% grey- 25% white

Incomplete DominanceIncomplete Dominance

• A cross between two organisms with different traits results in an offspring with a third phenotype that is a blending of the parental traits.

• It’s like mixing paints: – Red + White = Pink– Red does not totally block (dominate)

white, we end up with something in-between.

Inheritance in Inheritance in SnapdragonsSnapdragons

Let’s look at cattle….Let’s look at cattle….This cow resulted from a cross between a cow with red fur and a cow with white fur.

This is called ‘roan’ fur; red & white fur together.

CodominanceCodominance

• Similar to incomplete dominance in that there is a 3rd phenotype

• In COdominance, the “recessive” and “dominant” alleles appear together in the phenotype of hybrid organisms.

• Red x White = red & white

Polygenic (Multifactorial) Traits

• Phenotype is determine by more than one gene

• Often results in gradations, where each gene has an additive effectEx) If 10 gene loci are turned on plant will be 20cm tall, if only 5 loci are turned on plant will be 10cm tall

• Results in a bell-shaped curve – Skin color & Height are examples in humans

Phenotype Distribution:Polygenic Traits

Full color: CC, Ccch, Cch, or CcChinchilla: cchch, cchcch, or cchcHimalayan: chc, or chchAIbino: cc

KEY

C = full color; dominant to all other alleles

cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles

ch = Himalayan; color in certain parts of the body; dominant to c allele

c = albino; no color; recessive to all other alleles

Multiple Alleles• More than 2 alleles for a particular trait

ABO Blood Types

• In addition to having multiple alleles, ABO blood type also exhibits codominance

• ‘IA’ & ‘IB’ are codominant• ‘i’ is recessive

ABO Blood TypingABO Blood Typing

Genotype Blood Type

IA IA or IA i

IB IB or IB i

IAIB

ii

What does your ABO blood type mean?

Remember the ‘flags’ on our cell membranes? They help cells to recognize each other.

Some of those flags ‘announce’ your blood type. We call these flags antigens

Blood

Type

Antigen

A

B

AB

O

ABO Blood TransfusionsABO Blood Transfusions

Blood Type

Can receive:

A

B

AB*

O*

Summary of ABO Blood Types

Rh FactorRh Factor• Blood can also be categorized as + or –

• This refers to the presence (dominant) or absence (recessive) of the Rh antigen

Phenotype Genotype(s) Antigen

Rh+

Rh-

Chromosomal Theory of Inheritance• This theory states that genes occupy specific loci on

chromosomes and it is the chromosomes that undergo segregation and independent assortment during meiosis.

Gene Linkage & Mapping Chromosomes• According to the chromosomal theory of inheritance, genes on the

same chromosome are more likely to be inherited together• Crossing over helps to increased variation, but the closer two

genes are on a chromosome the more likely they are to be “linked”• The frequency of crossing over between two genes can be used to

estimate the relative positions of genes on chromosomes

Sex Chromosomes & Autosomes

• Recall that two of the 46 human chromosomes are known as sex chromosomes, because they determine the individual’s sex.– Females have two copies of an X chromosome.– Males have one X chromosome and one Y

chromosome.

• The remaining 44 chromosomes are known as autosomal chromosomes or autosomes.

Sex-Linked GenesSex-Linked Genes

• Located on one of the sex chromosomes

(X or Y)• Since the X

chromosome is longer, it has many genes not found on the Y chromosome.

• Most sex-linked genes are X-linked genes.

Sex-Linked Genes

X-Linked InheritanceX-Linked Inheritance• Examples:

– Hemophilia• The protein necessary for normal blood clotting is

missing

– Colorblindness• Defective version of one or all of the 3 genes

responsible for color vision

– Male Pattern Baldness• Hair loss

– Duchenne Muscular Dystrophy• Weakening and loss of skeletal muscle

• These traits are recessive & more common in males. Why?

Possible Inheritance of Colorblindness Allele

Complete the following sex-linked crosses:

Eva and Paul just had a son, Michael. Paul is has normal color vision, but Eva’s father was colorblind. What is the likelihood that Michael is colorblind?

Laura and Steve are expecting their first child. They are concerned about the chances their child might be hemophiliac because both Steve and Laura’s father are hemophiliac. What is the probability of Laura and Steve having a hemophiliac child?

Complete the following pedigrees. Which is for a sex-linked trait? How do you know?