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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

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Overview l.jpg

Human inheritance patterns:

  • Autosomal

  • Sex-linked

    Sex determination systems

Human chromosomes l.jpg
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

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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 l.jpg

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

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

Some of these alleles cause genetic disorders

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Genetic Abnormality inherited in simple Mendelian patterns

Rare / uncommon version of trait

Not life-threatening

e.g. polydactyly

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Genetic Disorder inherited in simple Mendelian patterns

Heritable condition

Mild to severe medical repercussions

Characterized by set of symptoms

= syndrome

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Autosomal Dominant Inheritance inherited in simple Mendelian patterns

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 l.jpg
Huntington’s Disease inherited in simple Mendelian patterns

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

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





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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 l.jpg
Autosomal Recessive Inheritance populations because:

Recessive allele

Must be homozygous recessive to express trait

If heterozygous for the trait = carrier

e.g. cystic fibrosis, sickle cell anemia

Cystic fibrosis l.jpg
Cystic Fibrosis populations because:

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 l.jpg
Sickle Cell Anemia populations because:

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

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If both parents are carriers (heterozygous), offspring has 50% chance of being carrier (heterozygous) & 25% chance of being affected (homozygous recessive)





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Sex Determination in Humans 50% chance of being carrier (heterozygous) & 25% chance of being affected (homozygous recessive)

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 l.jpg
The SRY Gene 50% chance of being carrier (heterozygous) & 25% chance of being affected (homozygous recessive)

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)

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XX embryo 50% chance of being carrier (heterozygous) & 25% chance of being affected (homozygous recessive)

= no Y, no SRY,  testosterone

= ovaries form

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

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The X Chromosome 50% chance of being carrier (heterozygous) & 25% chance of being affected (homozygous recessive)

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)

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X-Linked Inheritance 50% chance of being carrier (heterozygous) & 25% chance of being affected (homozygous recessive)

Thomas Hunt Morgan & Drosophila

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

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X-Linked Inheritance 50% chance of being carrier (heterozygous) & 25% chance of being affected (homozygous recessive)

Males show their only allele

Males inherit only from mother

Fathers pass their only allele to all daughters

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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

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Hemophilia A simple Mendelian inheritance

Bleeding disorder

(caused by lack of clotting factor)

Occurs primarily in males (1/10,000)

Severity varies

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Red-Green Colour Blindness simple Mendelian inheritance

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

Little or no perception of reds, greens, yellows

Affects ~10% of males

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Punnett Squares for X-Linked Crosses simple Mendelian inheritance





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

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Unaffected female & affected male simple Mendelian inheritance

Female offspring:

All carriers

Male offspring:

All normal

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Carrier female & normal male simple Mendelian inheritance

Female offspring:

0.5 carrier

0.5 normal

Male offspring:

0.5 normal

0.5 affected

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Female carrier & affected male simple Mendelian inheritance

Female offspring:

0.5 carrier

0.5 unaffected

Male offspring:

0.5 normal

0.5 affected

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More males than females affected simple Mendelian inheritance

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)

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Unaffected male simple Mendelian inheritance

Affected male

Unaffected female

Carrier female

Females are the bridge between generations of affected males

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Pedigrees simple Mendelian inheritance

Genetic connections among individuals

Info from several generations collected

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

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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

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A pedigree for a dominant trait Mendelian genetics







A pedigree for a recessive trait












How to read pedigrees

I, II, III = generations

= male

= female

= parents

= offspring

= shows trait


= does not show trait


= known carrier (heterozygote) for

recessive trait





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

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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 l.jpg
Y-Linked Inheritance or recessive allele?

Genes can only be passed from father to son

No effect from mother

No effect on daughters

e.g. hairy ear (pinna) syndrome

An example l.jpg
An Example or recessive allele?

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?

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Heritable Changes in Chromosome # or recessive allele?

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

Consequences can be minor or lethal

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Most changes in chromosome number occur because of or recessive allele?non-disjunction

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

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(a) Aneuploidy or recessive allele?

Normal # ± 1 chromosome

Usually fatal

Basis of most miscarriages

Chances of non-disjunction  with  maternal age

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e.g. Down Syndrome (Trisomy 21) or recessive allele?

Child inherits extra copy of chromosome 21

(2n for all other chromosomes)

1/900 births

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Distinct physical characteristics: or recessive allele?

Weak muscle tone, small mouth that can’t accommodate tongue, uniquely-shaped eyelids

Varying degrees of mental retardation

Often ↓ immune response, heart malformations

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(b) Polyploidy or recessive allele?

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

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e.g. polyploidy in plants or recessive allele?

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

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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

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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

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Changes in # of Sex Chromosomes asexually

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

Relatively frequent:

Often results in learning disabilities & speech problems

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Female Sex Chromosome Abnormalities asexually

Turner Syndrome

Trisomy X

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(a) Turner Syndrome asexually

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

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

↓ sex hormone production & non-functional ovaries

(2˚ sexual traits do not develop properly)

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asexually risk of cardiovascular disease, kidney defects, hearing loss

Display X-linked recessive disorders more frequently than XX women

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(b) Trisomy X asexually

Women with 1 extra X chromosome


1/1000 live births

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

Fertile adults

(usually bear normal XX & XY children)

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Male Sex Chromosome Abnormalities otherwise no detectable defects

Klinefelter Syndrome

XYY Syndrome

A klinefelter syndrome l.jpg
(a) Klinefelter Syndrome otherwise no detectable defects

Inherit extra X chromosome from mother


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)

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Syndrome develops during puberty: otherwise no detectable defects

  • Overweight, tall, small sex organs

  • Normal intelligence

    (some learning disabilities & short-term memory loss)

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  • Feminizing effects otherwise no detectable defects

  • because  testosterone &  estrogen

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

  • Testosterone injections can reverse female traits

  • B xyy males l.jpg
    (b) otherwise no detectable defectsXYY Males

    1/500-1/1000 males

    Taller than average, ↑ ↑ testosterone levels, severe acne

    Mild mental impairment

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    Many XXX, XXY, XYY children not even diagnosed otherwise no detectable defects

    = unfairly categorized as underachievers

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    Why are changes in sex chromosome number tolerated? otherwise no detectable defects

    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 l.jpg
    Sex Determination Systems otherwise no detectable defects

    XX / XY

    XX / XO

    ZZ /ZW

    # of chromosomes


    Xx xy l.jpg
    XX / XY otherwise no detectable defects

    e.g. mammals, fruit flies

    Female = XX

    Male = XY

    Male produces 2 types of sperm

    (one has X, other has Y)

    Sex is determined by sperm cell at fertilization

    Xx xo l.jpg
    XX / XO otherwise no detectable defects

    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 l.jpg
    ZZ / ZW otherwise no detectable defects

    e.g. some fish, butterflies, birds

    Female = ZW

    Male = ZZ

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

    Sex is determined by egg cell at fertilization

    Chromosome number l.jpg
    Chromosome Number otherwise no detectable defects

    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 l.jpg
    Hermaphrodites otherwise no detectable defects

    e.g. many plants & invertebrates

    Have both male & female sex organs

    All individuals in a species have same complement of chromosomes



    Banana slug

    Possess a mechanism against self-fertilization so only function as a single sex at a time

    Prefer sexual reproduction but will self-fertilize

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