Chapter 10 – Patterns of InheritanceWhy do we look like our parents?

What do we inherit from them?

History of genetics

Gregor Mendel

Austrian monk who in 1860 developed theories on inheritance

His organism of study was the pea plant

His theories came from the careful fertilization of these plants & looking at the characteristics of the offspring (color, seed texture, etc)

Then utilizing a little MATH, he came up with formulas & theories on how parents transmit characteristics to their offspring

The neat thing!

His theories came before our understanding of meiosis

Chapter 10 – Patterns of InheritanceMendel’s Laws

The law of segregation

Each individual has two factors for each trait

The factors segregate (separate) during the formation of gametes

Each gamete contains only one factor from each pair of factors

Fertilization gives each new individual two factors for each trait

The law of independent assortment

Each pair of factors segregates independently of the other pairs. Remember MEIOSIS

All possible combinations of factors can occur in the gametes

Before we look at these laws there are some terms that must first be clarified!

Chapter 10 – Patterns of InheritanceWhat is a gene?

A sequence of DNA that encodes for a trait

What is an allele?

An alternative form of a gene

Thus, if you have a hair color gene, you could have a brown allele & a blonde allele to determine brown vs. blonde hair

How are these alleles represented in your chromosome pairs?

As a pair, so if the hair color genes are located on chromosome #1, each of your chromosome #1s will have an allele for hair color

So if “B” = brown allele & “b” = blonde allele, your possible combinations are:

BB, Bb, bb = these are your genotypes = the genetic make-up for this trait

Thus if you are BB, you only have brown alleles, then your hair color must be BROWN = phenotype = expressed or visible trait linked to your genotype

Chapter 10 – Patterns of InheritanceBut what would your phenotype be if you were Bb?

Genotype terminology

Homozygous = the two alleles in a pair are identical

Heterozygous = the two alleles in a pair are different

Thus a Bb person has a heterozygous genotype, but what would his phenotype be?

Dominant vs. Recessive relationship between alleles

If a Bb person has brown hair, then the B allele is dominant to the b allele & the dominant phenotype will manifest itself in these heterozygous individuals

There are other allele relationships which we will discuss later in the chapter

Let’s look at some genetic problems to illustrate these concepts

Chapter 10 – Patterns of InheritanceWhat will be the composition of a parents alleles given the following composition?

BB Bb bb

Thus a parent gives up only one allele for a given trait to their offspring!

Chapter 10 – Patterns of InheritanceHow will the offspring inherit these traits & what will they look like?

One trait or monohybrid crosses – Utilization of the Punnett Square

Homozygous dominant X Homozygous recessive

Chapter 10 – Patterns of InheritanceHow will the offspring inherit these traits & what will they look like?

Heterozygous X Heterozygous

Chapter 10 – Patterns of InheritanceHow will the offspring inherit these traits & what will they look like?

Brown X Blonde

Chapter 10 – Patterns of InheritanceHow will two traits be transmitted from parents to offspring?

Two-trait cross or dihybrid cross – Utilization of Punnett Square

Chapter 10 – Patterns of InheritanceHow will two traits be transmitted from parents to offspring?

Two-trait cross or dihybrid cross – Utilization of Punnett Square

Chapter 10 – Patterns of InheritanceWould you want to use a Punnett Square for any crosses involving 3 or more traits?

The Product Rule

Chapter 10 – Patterns of InheritanceProduct Rule Problem #2

Chapter 10 – Patterns of InheritanceGenetic disorders

Autosomal dominant disorders

Homozygous dominant or heterozygous individuals will have the disorder

Huntington disease

Degenerative neurological disorder = brain cells die prematurely

1 in 20,000 individuals

Onset = middle age

Death = within 10 to 15 years

Genetic screening = available but would you want it!

Chapter 10 – Patterns of InheritanceGenetic disorders

Autosomal recessive disorders

Homozygous recessive individuals will have the disorder

Cystic fibrosis

1 in 20 Caucasians are carriers

1 in 2500 children born will have the disorder

Produces thick mucus in the lungs & pancreas, preventing proper functioning of theseorgans

Chapter 10 – Patterns of InheritanceBeyond Mendel’s Laws

Polygenic Inheritance

When one trait is governed by two or more sets of alleles

Skin color as an example

The number of pairs of alleles is not known but a simple two pair example can give light on how skin color variability can be attained.

Just count the total number of dominant alleles in each of the phenotypes below

Phenotype Genotype

Very Dark AABB

Dark AABb or AaBB

Medium Brown AaBb or AAbb or aaBB

Light Aabb or aaBb

Very Light aabb

Chapter 10 – Patterns of InheritanceMultiple Alleles & Degrees of dominance

ABO Blood Type

How many different blood types are there (disregard + and -)?

4, and they are A, B, AB, & O

How do you type blood?

Blood type is determined by the presence or absence of glycoproteins embedded in your red blood cell’s membrane

Thus if you are:

Type A blood you have type A glycoprotein

Type B blood you have type B glycoprotein

Type AB blood you have both glycoproteins

Type O blood you have neither glycoprotein

What are the alleles that determine blood type & what genotypes give the above mentioned phenotypes?

Chapter 10 – Patterns of InheritanceAlleles

IA, IB, i

Phenotypes Genotypes

A IA IA or IA i

B IB IB or IB i

AB IA IB

O ii

What is the relationship between the IA allele and the i allele?

What is the relationship between the IB allele and the i allele?

Dominant vs. Recessive

What is the relationship between the IA and IB alleles?

Co-dominance = both alleles within the allele pair are expressed equally

Chapter 10 – Patterns of InheritanceIncomplete Dominance

The heterozygote condition gives an intermediate phenotype

Sickle cell anemia

A genetic blood disorder characterized by sickle shaped red blood cells, which result in insufficient delivery of oxygen to the tissues

The cells sickle due to a defective hemoglobin gene

Hemoglobin is a protein within red blood cells which binds & delivers oxygen to the tissues

HbA = normal hemoglobin allele

HbS = sickle cell hemoglobin allele

Phenotype Genotype

Normal HbA HbA

Sickle cell trait HbA HbS

Sickel cell anemia HbS HbS

Chapter 10 – Patterns of InheritanceHow does the sickle cell trait fall in between the normal and sickle cell anemia phenotypes?

The sickle cell traits red blood cells look normal, but can sickle if the individuals become dehydrated or suffer mild oxygen deprivation

The neatest phenotypic difference is their resistance to malaria

In high malaria areas of Africa:

Sickle cell amenia babies die from sickle cell

Normal individuals risk dying from malaria infection

Sickle cell trait individuals are immune from contracting the malaria parasite and won’t die from sickle cell

Chapter 10 – Patterns of InheritanceSummary of Dominant vs. Recessive, Co-dominance, & Incomplete dominance

Let’s use a simple color analogy (Red allele vs. White allele) – symbols for each of the above relationships are different, but the key thing to remember is the homozygous condition vs. the heterozygous condition

Phenotype

Genotype Dominant Co-dominance Incomplete dominance

Homozygous red allele Red Red Red

Homozygous white allele White White White

Heterozygous Red Red & White Pink

The heterozygous condition is the only place that the co-dominant or incomplete dominant phenotype will express itself

Chapter 10 – Patterns of InheritancePRACTICE QUESTIONS

1. What are the laws of segregation & independent assortment?

2. What is a gene? What is an allele?

3. Differentiate between Dominance, Co-dominance, & Incomplete dominance

4. Why is blood typing an example of a multiple allele system?

5. Given that Tom is BB & Mary is bb, where B = big nosed and b = small nosed. What would be the phenotypes of their children & in what percentage?

6. Given that Bob is Bb and Diane is Bb, what would be the phenotypes of their children & in what percentage?

7. What kind of genotypes would Alan’s sperm have if he were BbTt?

8. If he married Sue who had a genotype of Bbtt, how many of their offspring would be bbtt? BbTt? BbTT?

9. Given that Tom is heterozygous for brown eyes & Mary is homozygous for brown eyes, if all of their children are brown eyed what is the relationship between the brown eye allele & the blue eye allele?