GENETICS

GENETICS

• The scientific study of heredity

• Heredity: the passing down of traits from parents to offspring via genes and chromosomes

Gregor Mendel

• Austrian monk in 1860’s

• Studied different traits of the garden pea

• Discovered the basic laws of genetics

Fertilization• BODY CELL: full

number of chromosomes (two copies = diploid)

• SEX CELL: half the number of chromosomes (one copy = haploid)

• FERTILIZATION: we get genes from each of our parents

Genes and Alleles• GENE: section of DNA that determines

a trait (Mendel called them factors)• ALLELE: particular form of a trait

(represented by letters)• EX: plant size

• “T” is the tall allele

• “t” is the short allele

Combinations of Alleles

• We get one allele from each parent

• Three possible combinations

– TT: homozygous dominant

– Tt: heterozygous

– tt: homozygous recessive

T T

T

T

TT x TT tt x tt

w

w

t t

Mendel generated “true breeding” (HOMOZYGOUS) plants by self-fertilizing tall plants and short plants. He called this his parental, or P1 generation.

Mendel Generates his P1 Generations

TT

TT

TT

TT tt tt

tttt

What have we learned so far?

• Punnet square - visual aid showing how traits are inherited in a cross (mating)

• Genotype - combination of alleles that an individual has. (TT, tt, Tt)

• Phenotype - physical appearance of a trait

When Mendel crossed a “true breeding”(homozygous) tall plant and a “true breeding”(homozygous) short plant, he found that all of the offspring were tall. He called this his First Filial or F1 generation.

T

T

t t

Mendel generates his F1 generation…

Tt

TtTt

Tt

What did Mendel learn?

• Law of dominance and Recessiveness: one allele in a pair may mask the other allele, preventing it from having an effect.

Dominant (tall) allele is capable of masking the recessive (short) allele

Recessive allele (short) is masked by the dominant (tall) allele.

When Mendel allowed the F1 generation to self-fertilize, he found that ¾ of the resulting plants were tall and ¼ were short (3:1 ratio). He called this his Second Filial or F2 generation.

T

t

T t

Mendel generates his F2 generation…

tt

TT Tt

Tt

What did Mendel learn?

• LAW OF SEGRIGATION: The two alleles separate during the formation of egg and sperm

• Offspring get one allele from each parent.

Think about meiosis…

• At what phase of meiosis do alleles separate?

ANAPHASE I

Three Laws• Law of Dominance and Recessiveness: one allele in a pair

may mask the other allele, preventing it from having an effect.

• Law of Segregation: two alleles separate during the formation of egg and sperm

• Law of Independent Assortment: alleles for different characteristics are distributed independently to reproductive cells

Patterns of Inheritance

• Dominant/Recessive

• Incomplete Dominance

• Codominance

• Multiple Alleles

• Sex-linked

Dominant/Recessive• One allele is dominant over the other

(capable of masking the recessive allele)

WW = purple ww = white Ww = purple

Problem: Dominant/Recessive• In pea plants, purple flowers (W) are dominant over

white flowers (w) show the cross between two heterozygous plants.

W

w

W w

ww

Ww

Ww

WW- WW (1); Ww (2); ww (1)- ratio 1:2:1

- purple (3); white (1)- ratio 3:1

GENOTYPES:

PHENOTYPES:

Incomplete Dominance• A third (new) phenotype appears in the

heterozygous condition.

• Flower Color in 4 O’clocks

RR = red R’R’ = white RR’ = pink

WORK IN GROUPS ON WORKSHEET

Problem: Incomplete Dominance• Show the cross between a pink and a white

flower.

- RR’ (2); R’R’ (2)- ratio 1:1

- pink (2); white (2)- ratio 1:1

R R’

R’

R’

R’R’RR’

R’R’RR’

GENOTYPES:

PHENOTYPES:

Codominance

• The heterozygous condition, both alleles are expressed equally

• Sickle Cell Anemia in Humans

SS = normal cells

S’S’ = sickle cells

SS’ = some of each

Problem: Codominance• Show the cross between an individual with

sickle-cell anemia and another who is a carrier but not sick.

S S’

S’

S’

SS’

SS’

S’S’

S’S’

- SS’ (2) S’S’ (2)- ratio 1:1

- carrier (2); sick (2)- ratio 1:1

GENOTYPES:

PHENOTYPES:

Multiple Alleles

• There are more than two alleles for a trait

• Blood type in humans

• Blood Types?– Type A, Type B, Type AB, Type O

• Blood Alleles?– A, B, O (in book – IA, IB, I)

Rules for Blood Type• A and B are codominant

– AA = Type A– BB = Type B– AB = Type AB

• A and B are dominant over O– AO = type A– BO = type B– OO = type O

Problem: Multiple Alleles• Show the cross between a mother who has type

O blood and a father who has type AB blood.

- AO (2) BO (2)- ratio 1:1

- type A (2); type B (2)- ratio 1:1

GENOTYPES:

PHENOTYPES:

A B

O

O

AO

BO

BO

AO

Problem: Multiple Alleles• Show the cross between a mother who is heterozygous for

type B blood and a father who is heterozygous for type A blood.

-AB (1); BO (1); AO (1); OO (1)- ratio 1:1:1:1

-type AB (1); type B (1) type A (1); type O (1)- ratio 1:1:1:1

GENOTYPES:

PHENOTYPES:

A O

B

O

AB

OO

BO

AO

KARYOTYPE

• SEX CHROMOSOMES: X and Y – determine the sex of an organism.– XX = female– XY = male

• AUTOSOMES: 22 pairs of chromosomes that do not determine sex.

•chart of the chromosomes an individual has.

Sex Linked Traits

• SEX LINKED TRAIT: found on the X chromosome

• AUTOSOMAL TRAIT: exists on any chromosome that does not determine the sex of the organism.

Color Blindness• Sex linked, recessive condition

• Alleles:

– Xc = color blind

– XC = normal vision

• Carrier: has a color blind allele but does not express the trait (has normal vision).

– XCXc

Problem: Sex linked• If a woman who is a carrier of the gene for color

blindness mates with a man with normal vision, what offspring will result?

•XCXC – female normal

•XCXc – femal carrier

•XCY – male normal

•XcY – male color blind

XC Xc

XC

Y

XCXC

XcY

XCXc

XCY

Pedigrees• A graphic representation of an individual’s

family tree

• Used to recognize patterns of inheritance

Pedigree Symbols

male – unaffected

male – affected

female – unaffected

female – affected

- marriage

- offspring