Chapter 11 - Introduction to Genetics

Gregor Mendel

Strong background in plant breeding and mathematics

Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

Traits Specific characteristics that vary

from one individual to another Determined by genes 7 traits studied by Mendel

Seed shape and color, pod shape and color, position, height

True BreedingProducing offspring with traits identical to parent Result of self-pollination of pea plantsHybridOffspring that results from parents with different traitsResult of cross-pollination

Mendel’s Peas

Genes Chemical factors that determine

traits (units of information) Passed from parents to offspring Each has a specific location (locus)

on a chromosome

Alleles

Different forms of a gene Dominant allele (Uppercase letter)

overrules a recessive allele (lowercase letter) that it is paired with

Principle of Dominance Some alleles are dominant and

others are recessive Recessive allele must be present

WITHOUT dominant allele, then recessive form of trait will be expressed

Allele Combinations

Homozygous =purebred having two identical alleles at a locus AA (dominant expressed) or aa

(recessive expressed) Heterozygous =hybrid

having two different alleles at a locus Aa (dominant expressed)

Segregation Separation of alleles during the

formation of gametes (sperm and egg)

Tracking Generations

Parental generation Pmates to produce

First-generation offspring F1

which mate to produce

Second-generation offspring F2

Genotype & Phenotype

Genotype refers to particular genes an individual carries (represented by letters)

Phenotype refers to an individual’s physical/observable traits: actual expression of gene

Cannot always determine genotype by observing phenotype…

What happens when a true breeding purple flower breeds with a true breeding white flower?

FilamentAnther

StigmaStyle

Ovary

Carpel

PetalSepalOvule

Stamen

Section 24-1

Figure 24–5 The Structure of a Flower

How did Mendel fertilize the

plants?

P1

F1

Mendel’s Question Mendel wanted to know if the

recessive alleles disappeared or are they still in the F1, just hidden.

Note differences between phenotype and genotype amongst different generations.

F M

Dominant trait is

expressed

Recessive appears

Type of alleles

The actual expression

Female

Male

Allele - different form of a gene

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants

P GenerationF1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Principles of Dominance

T T t t

P Generation F1 Generation

F2 Generation

Tall Short Tall TallTall Tall Tall Short

Principles of Dominance

T T t t T t T t

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Principles of Dominance

T T t t T t T t T T T t T t t t

Tt X Tt Cross

Tt X Tt Cross

What are the phenotypic and genotypic ratios?

Dominant & Recessive Alleles

If you have attached earlobes, you inherited two copies of the recessive allele (ee)

If you have detached earlobes, you may have either two or one copy of the dominant allele (EE or Ee)

Earlobe Variation

Whether a person is born with attached or detached earlobes; depends on a single gene

Gene has two forms (alleles)

Earlobe Variation

You inherited one allele for this gene from each parent

Dominant allele specifies detached earlobes (E)

Recessive allele specifies attached earlobes (e)

Incomplete Dominance

•Neither allele is dominant

over the other.

•Both alleles represented by

uppercase letters (RW).

•Combination of red and white making a new

phenotype (pink!)

Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

Codominance

•BOTH alleles are dominant (ex. Red flower and White flower made Red and White flower, not pink!)

•Blood Type A (IA) and Blood Type B (IB), when combined make Blood Type AB

Multiple Alleles Genes with more than 2 alleles Ex. With blood types there are 2

dominant alleles and 1 recessive IA=dominant Blood Type A

IB=dominant Blood Type B

i=recessive Blood Type O

Phenotype(Blood Type Genotype

Antigen on Red Blood Cell

Safe Transfusions

To From

Figure 14-4 Blood Groups

(Pg. 344-345)

ABO Blood Type:Allele Combinations

Type A - IAIA or IAi

Type B - IBIB or IBi

Type AB - IAIB

Type O - ii

Phenotypes: Genotypes:

Universal Acceptor

Universal Donor

What percentages of offspring do you get from a mother with blood type AB and a father with blood type O?

IAIB x i i

i i

IA

IB

What percentages of offspring do you get from a mother with blood type AB and a father with blood type O?

IAIB x i i

i i

IA

IB

IAi

IBi

IAi

IBi

Genetic Problem - Blood Types A man with A blood and a woman

with B blood have a child. What are the chances of their child having having B blood if they are both heterozygous? What are their chances of having three children with O blood?

Rh factor - Another Blood Trait Rh+ is dominant What happens when Rh- mother is

impregnated by Rh+ father? Baby is Rh+ and mother becomes

sensitized after first birth Mother then creates antibodies to

fight Rh+ blood Further Pregnancy complications

Polygenic Traits

More than one gene controls a trait Skin color more than one gene,

incomplete dominance Bell Curve-used to represent

frequency of polygenic traits

A, B, and C are dark a, b, and c are light