Chapters 9 & 12: Genetics

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

•The factors separate when the gametes (eggs & sperm) are formed –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 pairs of factors separate independently during meiosis – The Law of Independent Assortment

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

• Alleles - various forms of a trait

• Ex: tall and short height

curly and straight hair

brown and blue eyes

Widow’s peak

The Epicanthal Fold (eye fold)

Genotype and Phenotype

• Genotype refers to the genes of an individual; 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 appearance of the individual.

• 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 used to determine

probability

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

Probability

• Determine the odds of an event occurring. • 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).

• Codominance - both alleles are equally expressed in a heterozygote

• Ex: Blood type – AB blood

• Incomplete dominance – heterozygous genotype shows an intermediate phenotype, representing a blending of traits.

• 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:

Inheritance of blood type

Incomplete dominance

• Other examples of incomplete dominance:

• Plants called four o’clocks

RR – red

RR’ – pink

R’R’ – white

• So a cross

between two

pink plants produces

1 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, Fragile X Syndrome

Phenotype Gentoype

Normal female XBXB

Carrier female XBXb

Affected female

XbXb

Normal male XBY

Affected male XbY

Cross involving an X-linked allele

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

Amniocentesis

• Uses a needle to extract amniotic fluid from the uterus of a pregnant woman from the 14th to 17th week of pregnancy.

• Up to 400 chromosome and biochemical problems can be detected by culturing fetal cells that are in the amniotic fluid.

• 0.3% chance of miscarriage with this procedure – do this test only if certain risk factors are present.

Amniocentesis

Chorionic Villi Sampling (CVS)

• Uses a thin suction tube to sample chorionic cells from the placenta as early as the fifth week of pregnancy.• Chorionic cells are found in the placenta

• The cells do not have to be cultured, and karyotyping can be done immediately.

• 0.8% risk of miscarriage but can be performed earlier than amniocentesis.

Chorionic villi sampling