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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
Overview

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


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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


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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

a

a

A

a


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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


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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


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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)

A

a

A

a


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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


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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)


The x chromosome l.jpg
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

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


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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

I

I

I

I

I

I

A pedigree for a recessive trait

I

II

?

?

?

?

III

?

?

?

IV

How to read pedigrees

I, II, III = generations

= male

= female

= parents

= offspring

= shows trait

or

= does not show trait

or

= known carrier (heterozygote) for

recessive trait

or

?

?

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


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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?


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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


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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


A turner syndrome l.jpg
(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

=XXX

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


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(a) Klinefelter Syndrome otherwise no detectable defects

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)


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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

    Hermaphrodites


    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

    Earthworm

    Lily

    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