Patterns in Inheritance

Chapter 10

What you absolutely need to

know

• Punnett Square with monohybrid and

dihybrid cross

• Heterozygous, homozygous, alleles,

locus, gene

• Test cross, P, F1, F2

• Mendel and his work

• Patterns of human inheritance

Early Ideas about Heredity

• People knew that sperm and eggs

transmitted information about traits

• Blending theory

• Problem:

– Would expect variation to disappear

– Variation in traits persists

No Blending Involved

• We cannot say that a red flower crossed

with a what flower produces a pink

flower.

Name this man

Gregor Mendel

• Strong background in plant breeding and

mathematics

• Using pea plants, found indirect but

observable evidence of how parents

transmit genes to offspring

Genetic Terms

A pair of homologous

chromosomes

A gene locus

A pair of alleles

Three pairs of genes

The Garden Pea Plant

• Self-pollinating

• True breeding (different alleles

not normally introduced)

• Can be experimentally cross-

pollinated

Impact of Mendel’s Work

• Mendel presented his results in 1865

• Paper received little notice

• Mendel discontinued his experiments in

1871

• Paper rediscovered in 1900 and finally

appreciated

Genes

• Units of information about specific traits

• Passed from parents to offspring

• Each has a specific location (locus) on a

chromosome

Alleles

• Different molecular forms of a gene

• Arise by mutation

• Dominant allele masks a recessive

allele that is paired with it

Allele Combinations

• Homozygous

– having two identical alleles at a locus

– AA or aa

• Heterozygous

– having two different alleles at a locus

– Aa

Genotype & Phenotype

• Genotype refers to particular genes an individual carries

• Phenotype refers to an individual’s observable traits

• Cannot always determine genotype by observing phenotype. Ex. A blond haired person can produce a red-haired offspring

Tracking Generations

• Parental generation P

mates to produce

• First-generation offspring F1

mate to produce

• Second-generation offspring F2

Monohybrid Crosses

• Use F1 offspring of parents that breed

true for different forms of a trait:

(AA x aa = Aa)

• The experiment itself is a cross between

two identical F1 heterozygotes, which

are the “monohybrids” (Aa x Aa)

F1 Results of One

Monohybrid Cross

Punnett Square of a

Monohybrid Cross

Female gametes

Male

gametes

Dominant

phenotype

can arise 3

ways,

recessive

only 1

a A

aa Aa

Aa AA A

a

F2 Results of

Monohybrid Cross

Testcross

• Individual that shows dominant

phenotype is crossed with individual

with recessive phenotype (AA x aa)

• Examining offspring allows you to

determine the genotype of the dominant

individual

Dominance Relations

• Complete dominance

• Incomplete dominance

– Heterozygote phenotype is somewhere

between that of two homozyotes

• Codominance

– Non-identical alleles specify two

phenotypes that are both expressed in

heterozygotes

F1 Results of Mendel’s

Dihybrid Crosses

• All plants displayed the dominant form

of both traits: AaBb

• We now know:

– All plants inherited one allele for each trait

from each parent

– All plants were heterozygous (AaBb)

F1 Results of Mendel’s

Dihybrid Crosses

• All plants displayed the dominant form

of both traits: AaBb

• We now know:

– All plants inherited one allele for each trait

from each parent

– All plants were heterozygous (AaBb)

Dihybrid Cross

Experimental cross between individuals

that are homozygous for different

versions of two traits

Now we want to try two traits

Purple AA

Tall BB

• Parent with purple flowers and and long

stems = AABB

• Purple AA

Tall BB

• Parent with white flowers and short

stems

• White aa and short stems bb

• Dihybrid cross is AABB x aabb

Phenotypic Ratios in F2

Four Phenotypes:

– Tall, purple-flowered (9/16)

– Tall, white-flowered (3/16)

– Dwarf, purple-flowered (3/16)

– Dwarf, white-flowered (1/16)

AaBb X AaBb

AaBb x AaBb produces the

following gametes

If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible

Down Syndrome

An extra Chromosome 21

A karyotype, performing a

genetic analysis

A female with Down Syndrome

Genes

• Units of information about heritable

traits

• In eukaryotes, distributed among

chromosomes

• Each has a particular locus

– Location on a chromosome

Homologous Chromosomes

• Homologous autosomes are identical in

length, size, shape, and gene sequence

• Sex chromosomes are nonidentical but

still homologous

• Homologous chromosomes interact,

then segregate from one another during

meiosis

Homologous Chromosomes

Crossing over with non sisters

Alleles

• Different molecular forms of a gene

• Arise through mutation

• Diploid cell has a pair of alleles at each

locus

• Alleles on homologous chromosomes

may be same or different

Sex Chromosomes

• Discovered in late 1800s

• Mammals, fruit flies

– XX is female, XY is male

• Human X and Y chromosomes function

as homologues during meiosis

Sex Determination

XX

XY

XX

XY

X X

Y

X

sex chromosome combinations possible

in new individual

Y

X

sperm

X

X

eggs

Female germ cell Male germ cell

The Y Chromosome

• Fewer than two dozen genes identified

• One is the master gene for male sex

determination

– SRY gene (sex-determining region of Y)

• SRY present, testes form

• SRY absent, ovaries form

Effect of Y

Chromosome

10 weeks

Y

pre

sen

t

Y

absent

7 weeks

birth approaching

appearance of structures

that will give rise to

external genitalia

appearance of

“uncommitted” duct system

of embryo at 7 weeks

Y

present

Y

absent

testis

ovary

testes ovaries

The X Chromosome

• Carries more than 2,062 genes

• Most genes deal with nonsexual traits

• Genes on X chromosome can be

expressed in both males and females

Karyotype Preparation -

Stopping the Cycle

• Cultured cells are arrested at

metaphase by adding colchicine

• This is when cells are most condensed

and easiest to identify

Karyotype Preparation

• Arrested cells are broken open

• Metaphase chromosomes are fixed

and stained

• Chromosomes are photographed

through microscope

• Photograph of chromosomes is cut

up and arranged to form karyotype

diagram

Human Karyotype

Crossover Frequency

Proportional to the distance that

separates genes

Crossing over will disrupt linkage between

A and B more often than C and D

Human Genetic Analysis

• Geneticists often gather information

from several generations to increase the

numbers for analysis

• If a trait follows a simple Mendelian

inheritance pattern they can be

confident about predicting the

probability of its showing up again

Pedigree

• Chart that shows genetic connections

among individuals

• Standardized symbols

• Knowledge of probability and Mendelian

patterns used to suggest basis of a trait

• Conclusions most accurate when drawn

from large number of pedigrees

Pedigree for Nicholas of

Russia

Genetic Disorder

• A rare, uncommon version of a trait

• Polydactyly

– Unusual number of toes or fingers

– Does not cause any health problems

– View of trait as disfiguring is subjective

Genetic Disorder

• Inherited conditions that cause mild to

severe medical problems

• Why don’t they disappear?

– Mutation introduces new rare alleles

– In heterozygotes, harmful allele is masked,

so it can still be passed on to offspring

Autosomal

Dominant Inheritance

• Trait typically

appears in

every

generation

• Achhondroplasi

a

• Huntington’s

Achondroplasia

• Autosomal dominant allele

• In homozygous form usually leads to

stillbirth

• Heterozygotes display a type of dwarfism

• Have short arms and legs relative to other

body parts

Huntington Disorder

• Autosomal dominant allele

• Causes involuntary movements,

nervous system deterioration, death

• Symptoms don’t usually show up until

person is past age 30

• People often pass allele on before they

know they have it

Woody Guthrie

Autosomal Recessive

Inheritance Patterns

• If parents are

both

heterozygous,

child will have a

25% chance of

being affected

• Galactosemia

• Caused by autosomal recessive allele

• Cannot manufacture an ezyme. Lethal

• Albinism, cystic fibrosis, sickle cell anemia

X-Linked Recessive

Inheritance

• Males show

disorder more

than females

• Son cannot inherit

disorder from his

father

Examples of X-Linked Traits

• Color blindness

– Inability to distinguish

among some of all colors

• Hemophilia

– Blood-clotting disorder

– 1/7,000 males has allele for hemophilia A

– Was common in European royal families

Aneuploidy

• Individuals have one extra or less

chromosome

• (2n + 1 or 2n - 1)

• Major cause of human reproductive

failure

Nondisjunction

n + 1

n + 1

n - 1

n - 1 chro

moso

me

align

ments

at

meta

phase

I

nondis

junctio

n at

anaph

ase I

align

ments

at

meta

phase

II

anap

hase

II

chromos

ome

number

in

gametes

Down Syndrome

• Trisomy of chromosome 21

• Mental impairment and a

variety of additional defects

• Can be detected before birth

• Risk of Down syndrome increases

dramatically in mothers over age 35