Chapter 12: Mendel and

Heredity

Heredity – The passing of traits from parents to offspring

Genetics – The study of heredity

Gregor Mendel • Austrian monk• Bred pea plants• 1860 - developed laws

of heredity

•He cross-pollinated plantsmms://204.13.204.36/Video9/mendelslaw.asf

•He bred plants to be pure for certain traits•Ex: Tall parent tall offspring

Short parent short offspring

•Then he cross-bred plants with opposite traits

•Tall x short•Round x wrinkled•Yellow x green

•Parents - P generation – tall x short•Offspring – F1 generation (1st filial generation)

•All offspring were tall (short trait disappeared)

•Allowed F1 generation to self-pollinate•F2 generation – 75% tall to 25% short(short trait reappeared)

•Repeated many times – always same ratios for each generation (see results slide #2)

Mendel’s Conclusions:•There are 2 factors for every trait (today we know these factors to be genes – 1 from mother, 1 from father)•One of these factors can be dominant over the other (the recessive trait)

•This is known as the Law of Dominance

• Alleles – each version of a gene• Ex: tall and short height

curly and straight hairbrown and blue eyes

What is the relationship between chromosomes, DNA, genes, and alleles?

•Chromosomes contain DNA and protein•DNA contains genes – each gene always has the same position on a chromosome (locus)•Alleles are different forms of the same gene (right vs. left-handed, blue vs. brown eyes)

Widow’s peak

•When an organism produces gametes, each pair of alleles is separated and each gamete has an equal chance of receiving either one of the alleles –The Law of Segregation•Each gamete only has 1 factor from each pair (haploid)

•Fertilization gives each new individual 2 factors again (diploid)

•Mendel then crossed pure plants that differed in 2 traits

•Ex: yellow, round peas crossed with green, wrinkled peas•F1 generation always showed dominant traits•F2 generation had the following results: (see next slide)

F2: 9 yellow, round

3 yellow, wrinkled

3 green, round1 green,

wrinkled

•Based on these results, Mendel concluded that during gamete formation, the alleles of each gene segregate independently – The Law of Independent Assortment

•Ex: Below, hairline and finger length are not dependent on each other

•Mendel’s Law of Independent Assortment would not hold true if the genes for two traits are located on the same chromosome – linked genes

Genotype and Phenotype

• Genotype refers to the set of alleles that an individual has for a character; can be represented by two letters • Homozygous - both alleles are the same

• Homozygous dominant - WW • Homozygous recessive - ww

• Heterozygous – alleles are different - Ww

• Phenotype refers to the trait that results from a set of alleles. • Both WW and Ww result in widow’s

peak, the dominant trait• ww will result in no widow’s peak, the

recessive trait

Monohybrid Crosses

• Considers only one trait. • Punnett square – chart that shows all of

the genotypes that could result from a given cross

•Ratio shows # of offspring with dominant vs. recessive trait

Probability

• The likelihood that a specific event will occur. • Expressed as fraction or percentage

• Ex: (1/4) or 25%• The probability that two or more independent

events will occur together is the product of their chances occurring separately• Ex: odds of having a boy = ½

Odds of having 2 boys = (1/2) x (1/2) = (1/4)

• The chance of widow’s peak:• WW or Ww = 75% or ¾

• Chance of a continuous hairline:• ww = 25% or 1/4

•Odds of having 3 children with a continuous hairline:

•(1/4) x (1/4) x (1/4) = (1/64)

Dihybrid Cross• Two traits are considered• Genotypes of the parents require four letters

(two for each trait).

Multiple Allelic Traits

• More than two alternative alleles exist for a particular trait.• Ex: blood type – A, B, and O are the 3

alleles• Each individual inherits only two alleles

for these genes.

• Codominance - both alleles for the same gene are fully expressed in a heterozygote• Ex: Blood type – AB blood

• Incomplete dominance – An offspring has a phenotype that is intermediate between the traits of its parents.• Ex: Curly, wavy, or straight hair in

Caucasians

ABO Blood Types• How your book shows blood type:

Blood type (phenotype)

Genotype

A IAIA or IA i

B IBIB or IBi

AB IAIB

O ii

Blood type (phenotype)

Genotype

A AA or AO

B BB or BO

AB AB

O OO

•How your teacher shows blood type:

Blood type Donor for Recipient from

A A, AB A, O

B B, AB B, O

AB AB A, B, AB, O

O A, B, AB, O O

Inheritance of blood type

Incomplete dominance

• Other examples of incomplete dominance:• Plants called four o’clocks

RR – redRR’ – pinkR’R’ – white

• So a crossbetween twopink plants produces1 red, 2 pink, and 1 white plant

RR RR’

RR’ R’R’

R R’

R

R’

• Another example includes Sickle cell disease in humans

• HbA represents normal hemoglobin; and HbS represents the sickled condition– HbAHbA – normal– HbSHbS – sickle-cell disease – HbAHbS - have the intermediate condition

called sickle-cell trait. • Heterozygotes have an advantage in malaria-

infested Africa because the pathogen for malaria cannot exist in their blood cells.

Sex determination:•Female – XX•Male – XY•Always 50% chance of having a boy or a girl•Male determines gender of baby

XX XX

XY XY

X X

X

Y

Sex-Linked Traits• Traits controlled by genes on the X or Y

chromosomes • X-linked or Y-linked

• Most X-linked traits are recessive, so a female would have to have two recessive genes to express the trait; a male would only need one.

• Y-linked traits are only passed from father to son

• Examples of X-linked traits include Color blindness, Hemophilia, & Muscular Dystrophy

Phenotype GentoypeNormal female XBXB

Carrier female XBXb

Affected female

XbXb

Normal male XBYAffected male XbY

Cross involving an X-linked allele

Sex Influenced Traits• Sex-influenced traits are autosomal traits that are

influenced by sex. If a male has one recessive allele, he will show that trait, but it will take two recessive for the female to show that same trait. One such gene is baldness.

• BB normal male & female• Bb bald male; normal female• bb bald male; bald female

Pedigree Charts• Constructed to show the pattern of

inheritance of a characteristic within a family.

• The particular pattern indicates the manner in which a characteristic is inherited (suggests X-linked, dominant, etc.)

Normal female

Carrier female

Affected female

Normal male

Affected male

Symbols used:

Male can be a carrier for an autosomal trait.

Autosomal recessive pedigree chart

Autosomal dominant pedigree chart