Human Inheritance

Chapter 9

Overview

Human inheritance patterns:

– Autosomal

– Sex-linked

Sex determination systems

Human Chromosomes

Humans: male & female, 2n

23 pairs of homologous chromosomes in cells

Each pair is structurally identical except sex chromosomes

(Female XX, male XY)

Autosomes are same in both sexes

Human X & Y chromosomes differ in appearance & genes

Have small region that allows to act like homologues during meiosis

Inheritance of sex chromosomes in certain combos determines gender

Karyotyping

Individual’s metaphase chromosomes organized by length, shape, centromere

location, etc.

Can detect abnormalities in chromosome structure or altered

chromosome # by comparing individual’s karyotype against species

standard

Most traits come from autosomal dominant / recessive alleles inherited in simple

Mendelian patterns

Some of these alleles cause genetic disorders

Genetic Abnormality

Rare / uncommon version of trait

Not life-threatening

e.g. polydactyly

Genetic Disorder

Heritable condition

Mild to severe medical repercussions

Characterized by set of symptoms

= syndrome

Autosomal Dominant Inheritance

Dominant allele

Trait usually appears each generation because allele is expressed in homozygous dominants

& heterozygotes

Remember: phenotypic ratio 3:1

e.g. Huntington’s disease, lactose intolerance

Huntington’s Disease

Degeneration of neurons in brain

Affects 1/20,000 – 1/1,000,000 people

Results in uncontrolled movements, emotional problems, loss of brain function

Symptoms include mood swings, difficulty making decisions & retaining info

No cure

If 1 parent is heterozygous & other is homozygous recessive, offspring has 50% chance of being

heterozygous

Aa Aa

aa aa

A

a

a

a

Some dominant alleles that cause severe problems persist in populations because:

• Expression of allele doesn’t affect reproduction

• Affected individuals reproduce before symptoms are evident

• Spontaneous mutations

Autosomal Recessive Inheritance

Recessive allele

Must be homozygous recessive to express trait

If heterozygous for the trait = carrier

e.g. cystic fibrosis, sickle cell anemia

Cystic Fibrosis

Production of very thick, sticky mucus

Affects lungs & digestive system(clogs lungs & hampers pancreas from breaking

down & absorbing food)

~30,000 people in US are affected

Average lifespan = 35-40 years

Sickle Cell Anemia

Body produces abnormally-shaped RBCs

= break down prematurely & cause anemia

Affects 1/500 African-Americans

If only 1 allele = sickle cell trait– 1/12 African-Americans have trait– Resistance to malaria

If both parents are carriers (heterozygous), offspring has 50% chance of being carrier

(heterozygous) & 25% chance of being affected (homozygous recessive)

AA Aa

Aa aa

A

A

a

a

Sex Determination in Humans

Every normal female egg has 1 X chromosome

½ of sperm cells have X, ½ have Y

Sperm that fertilizes egg determines gender

The SRY Gene

1 of 255 Y chromosome genes

Master gene for male sex determination

When expressed in XY embryos, initiates testes formation

Testes produce testosterone(controls expression of male 2 sexual traits)

XX embryo

= no Y, no SRY, testosterone

= ovaries form(make estrogens & other sex hormones that control expression of female 2 sexual traits)

The X Chromosome

1141 genes:

Some associated with sexual traits e.g. distribution of body hair & fat

Most of genes associated with non-sexual traits expressed in both males & females

(because males get 1 X chromosome)

X-Linked Inheritance

Thomas Hunt Morgan & Drosophila

Determined that genes for non-sexual traits are located on X chromosome

X-Linked Inheritance

Males show their only allele

Males inherit only from mother

Fathers pass their only allele to all daughters

X chromosome alleles result in phenotypes that follow simple Mendelian inheritance

Many recessive alleles cause genetic disorders

e.g. hemophilia A, red-green colour blindness

Hemophilia A

Bleeding disorder

(caused by lack of clotting factor)

Occurs primarily in males (1/10,000)

Severity varies

Red-Green Colour Blindness

Impairment or loss of function in light-sensitive cone cells in eyes

Little or no perception of reds, greens, yellows

Affects ~10% of males

Punnett Squares for X-Linked Crosses

XA Xa

XA

Y

Set up in much the same way as regular Punnett Squares, but use X & Y to represent sex chromosomes with superscript

letters to represent the alleles carried on those chromosomes

XA XA XA Xa

XAY XaY

Unaffected female & affected male

Female offspring:

All carriers

Male offspring:

All normal

Carrier female & normal male

Female offspring:

0.5 carrier

0.5 normal

Male offspring:

0.5 normal

0.5 affected

Female carrier & affected male

Female offspring:

0.5 carrier

0.5 unaffected

Male offspring:

0.5 normal

0.5 affected

More males than females affected

Heterozygous females have dominant allele on other X that masks recessive allele’s

effects

Males only have 1 X chromosome

(no 2nd X chromosome to counteract effects of recessive allele)

Females are the bridge between generations of affected males

Unaffected male

Affected male

Unaffected female

Carrier female

Pedigrees

Genetic connections among individuals

Info from several generations collected

Can predict probability of trait being expressed as well as trace trait origins backwards

Basic procedure is to create a family tree &

apply Mendelian genetics

Can’t assume that an individual has a trait or is a carrier without

evidence

A pedigree for a dominant trait

A pedigree for a recessive trait

I

II

III

I

II

III

IV

? ? ? ?

? ? ?

How to read pedigreesI, II, III = generations

= male = female

= parents

= offspring

or = shows trait

or = does not show trait

or = known carrier (heterozygote) for recessive trait

or ? ?

Common Symbols Used in Pedigrees

Notice you can use parents to determine children’s genotypes or children to

determine parents’.

= cannot determine genotype from pedigree

Looking at this pedigree, is the trait caused by a dominant or recessive allele?

How do you know?

Can you tell anything about the genotypes of these individuals?

Y-Linked Inheritance

Genes can only be passed from father to son

No effect from mother

No effect on daughters

e.g. hairy ear (pinna) syndrome

In cats, coat colour is determined by an X-linked gene. The black allele causes black coat colour while the other allele, orange, causes orange colour, but in

heterozygotes the cats are tortoiseshell (patches of black & orange).

This is an example of what type of inheritance?

What kind of offspring would you expect from a black female & an orange male?

An Example

Heritable Changes in Chromosome #

Chance events occur before or after cell division that result in wrong chromosome #

Consequences can be minor or lethal

Most changes in chromosome number occur because of non-disjunction

= 1 pair of chromosomes do not separate during mitosis or meiosis

(a) Aneuploidy

Normal # ± 1 chromosome

Usually fatal

Basis of most miscarriages

Chances of non-disjunction with maternal age

e.g. Down Syndrome (Trisomy 21)

Child inherits extra copy of chromosome 21

(2n for all other chromosomes)

1/900 births

Distinct physical characteristics:Weak muscle tone, small mouth that can’t accommodate

tongue, uniquely-shaped eyelids

Varying degrees of mental retardation

Often ↓ immune response, heart malformations

(b) Polyploidy

Cells have ≥ 3 of each type of chromosome (e.g. 3n, 4n, etc.)

Many angiosperms, insects, fish, animals are actually polyploid

Responsible for evolution via speciation

e.g. polyploidy in plants

Fertilized diploid egg duplicates chromosomes but fails to divide = tetraploid (4n)

Produce diploid gametes that can fuse with other diploid gametes = 4n offspring

Can self-fertilize or interbreed with other 4n individuals of same species

If breed with 2n individual from original species, offspring is triploid

(sterile because meiosis fails)

4n & 2n of original species can’t interbreed successfully

= new species can form in 1 generation

Polyploidy is common in plants because they can reproduce asexually

If a 4n animal was produced, it would have to mate with a 2n individual

↓

All 3n offspring would be sterile↓

= no speciation occurs

Non-disjunction causes most of changes in # of X & Y chromosomes

Relatively frequent:

Often results in learning disabilities & speech problems

Changes in # of Sex Chromosomes

Female Sex Chromosome Abnormalities

Turner Syndrome

Trisomy X

(a) Turner Syndrome

1 X chromosome; no corresponding X or Y

= XO

Affects 1/2500-1/10,000 newborn females

(75% because of non-disjunction from father)

98% of embryos spontaneously abort

Generally, XO females are 4’8” but well-proportioned

↓ sex hormone production & non-functional ovaries

(2˚ sexual traits do not develop properly)

↑ risk of cardiovascular disease, kidney defects, hearing loss

Display X-linked recessive disorders more frequently than XX women

(b) Trisomy X

Women with 1 extra X chromosome

=XXX

1/1000 live births

Some learning disabilities & taller than average, but otherwise no detectable defects

Fertile adults

(usually bear normal XX & XY children)

Male Sex Chromosome Abnormalities

Klinefelter Syndrome

XYY Syndrome

(a) Klinefelter Syndrome

Inherit extra X chromosome from mother

= XXY

1/500 -1/1000 males

• 2/3 from non-disjunction at meiosis• Other 1/3 because Y chromosome fails to separate

at mitosis

(XY in some cells, XXY in others)

Syndrome develops during puberty:– Overweight, tall, small sex organs

– Normal intelligence(some learning disabilities & short-term memory loss)

Feminizing effectsbecause testosterone & estrogen

( sperm count, sparse hair, high voice, enlarged breasts)

Testosterone injections can reverse female traits

(b) XYY Males

1/500-1/1000 males

Taller than average, ↑ ↑ testosterone levels,

severe acne

Mild mental impairment

Many XXX, XXY, XYY children not even diagnosed

= unfairly categorized as underachievers

Why are changes in sex chromosome number tolerated?

In females, one X chromosome is shut off

Coils up chromosome so can’t be transcribed

So … any extra X chromosomes get turned off

Remember X-chromosome inactivation?

Sex Determination Systems

XX / XY

XX / XO

ZZ /ZW

# of chromosomes

Hermaphrodites

XX / XY

e.g. mammals, fruit flies

Female = XXMale = XY

Male produces 2 types of sperm

(one has X, other has Y)

Sex is determined by sperm cell at fertilization

XX / XO

e.g. some insects

Female = XX

Male = XO

Male produces 2 types of sperm (one type bears X, other has no sex chromosome)

Sex is determined by sperm cell at fertilization

ZZ / ZW

e.g. some fish, butterflies, birds

Female = ZWMale = ZZ

Female produces 2 types of egg (one type has Z, other has W)

Sex is determined by egg cell at fertilization

Chromosome Number

e.g. most ants & bees

Have no sex chromosomes

Sex determined by # of chromosomes:

Female is 2n & comes from fertilized egg

Male is n & comes from unfertilized egg

Hermaphrodites

e.g. many plants & invertebrates

Have both male & female sex organs

All individuals in a species have same complement of chromosomes

Possess a mechanism against self-fertilization so

only function as a single sex at a time

Both stamen (male) & pistil (female) found on same

flower

Prefer sexual reproduction but will

self-fertilize

Earthworm

Banana slug

Lily