Chapter 15 the chromosomal basis of inheritance
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Chapter 15~ The Chromosomal Basis of Inheritance. The Chromosomal Theory of Inheritance. Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment. Chromosomal Linkage. Morgan Drosophilia melanogaster XX (female) vs. XY (male)

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Chapter 15 the chromosomal basis of inheritance

Chapter 15~ The Chromosomal Basis of Inheritance


The chromosomal theory of inheritance
The Chromosomal Theory of Inheritance

  • Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment


Chromosomal linkage
Chromosomal Linkage

  • Morgan

  • Drosophilia melanogaster

  • XX (female) vs. XY (male)

  • Sex-linkage: genes located on a sex chromosome

  • Linked genes: genes located on the same chromosome that tend to be inherited together


Classes of chromosomes
Classes of chromosomes

autosomalchromosomes

sexchromosomes


Discovery of sex linkage
Discovery of sex linkage

true-breeding

red-eye female

true-breeding

white-eye male

X

P

Huh!Sex matters?!

100%

red eye offspring

F1

generation

(hybrids)

100%

red-eye female

50% red-eye male

50% white eye male

F2

generation


What s up with morgan s flies
What’s up with Morgan’s flies?

x

x

RR

rr

Rr

Rr

r

r

R

r

R

Rr

Rr

R

RR

Rr

Doesn’t workthat way!

R

r

Rr

Rr

Rr

rr

100% red eyes

3 red : 1 white


Genetics of sex
Genetics of Sex

  • In humans & other mammals, there are 2 sex chromosomes: X & Y

    • 2X chromosomes

      • develop as a female: XX

      • gene redundancy,like autosomal chromosomes

    • an X & Y chromosome

      • develop as a male: XY

      • no redundancy

X

Y

X

XX

XY

XX

XY

X

50% female : 50% male


Let s reconsider morgan s flies
Let’s reconsider Morgan’s flies…

x

x

XRXR

XrY

XRXr

XRY

Xr

Y

XR

Y

XR

XR

XRXr

XRY

XRXR

XRY

BINGO!

XR

Xr

XRXr

XRY

XRXr

XrY

100% red females

50% red males; 50% white males

100% red eyes


Genetic recombination
Genetic recombination

  • Crossing over Genes that DO NOT assort independently of each other

  • Genetic maps The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency

  • Linkage maps Genetic map based on recombination frequencies


Genes on sex chromosomes
Genes on sex chromosomes

  • Y chromosome

    • few genes other than SRY

      • sex-determining region

      • master regulator for maleness

      • turns on genes for production of male hormones

        • many effects = pleiotropy!

  • X chromosome

    • other genes/traits beyond sex determination

      • mutations:

        • hemophilia

        • Duchenne muscular dystrophy

        • color-blindness


Human sex linkage
Human sex-linkage

  • SRY gene: gene on Y chromosome that triggers the development of testes

  • Fathers= pass X-linked alleles to all daughters only (but not to sons)

  • Mothers= pass X-linked alleles to both sons & daughters

  • Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia


Hemophilia

XHXH

XHXh

XHXh

XHXh

XH

XHY

XHY

XhY

Xh

male / sperm

XH

Y

XHXH

XHY

XHY

XH

female / eggs

Xh

XhY

XHXh

sex-linked recessive

Hemophilia

Hh x HH

XH

Y

carrier

disease


X inactivation

XH

XHXh

Xh

X-inactivation

  • Female mammals inherit 2 X chromosomes

    • one X becomes inactivated during embryonic development

      • condenses into compact object = Barr body

      • which X becomes Barr body is random

        • patchwork trait = “mosaic”

patches of black

tricolor catscan only befemale

patches of orange


Human sex linkage1
Human sex-linkage

  • X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)


Errors of Meiosis

Chromosomal Abnormalities


Chromosomal abnormalities
Chromosomal abnormalities

  • Incorrect number of chromosomes

    • nondisjunction

      • chromosomes don’t separate properly during meiosis

    • breakage of chromosomes

      • deletion

      • duplication

      • inversion

      • translocation


Nondisjunction

n

n

Nondisjunction

  • Problems with meiotic spindle cause errors in daughter cells

    • homologous chromosomes do not separate properly during Meiosis 1

    • sister chromatids fail to separate during Meiosis 2

    • too many or too few chromosomes

2n

n-1

n+1


Alteration of chromosome number
Alteration of chromosome number

error in Meiosis 1

error in Meiosis 2

all with incorrect number

1/2 with incorrect number


Nondisjunction1
Nondisjunction

  • Baby has wrong chromosome number~ aneuploidy

    • trisomy

      • cells have 3 copies of a chromosome

    • monosomy

      • cells have only 1 copy of a chromosome

n-1

n

n+1

n

monosomy

2n-1

trisomy

2n+1


Human chromosome disorders
Human chromosome disorders

  • High frequency in humans

    • most embryos are spontaneously aborted

    • alterations are too disastrous

    • developmental problems result from biochemical imbalance

      • imbalance in regulatory molecules?

        • hormones?

        • transcription factors?

  • Certain conditions are tolerated

    • upset the balance less = survivable

    • but characteristic set of symptoms = syndrome


Down syndrome
Down syndrome

  • Trisomy 21

    • 3 copies of chromosome 21

    • 1 in 700 children born in U.S.

  • Chromosome 21 is the smallest human chromosome

    • but still severe effects

  • Frequency of Down syndrome correlates with the age of the mother


Sex chromosomes abnormalities
Sex chromosomes abnormalities

  • Human development more tolerant of wrong numbers in sex chromosome

  • But produces a variety of distinct syndromes in humans

    • XXY = Klinefelter’s syndrome male

    • XXX = Trisomy X female

    • XYY = Jacob’s syndrome male

    • XO = Turner syndrome female


Klinefelter s syndrome
Klinefelter’s syndrome

  • XXY male

    • one in every 2000 live births

    • have male sex organs, but are sterile

    • feminine characteristics

      • some breast development

      • lack of facial hair

    • tall

    • normal intelligence



Jacob s syndrome male
Jacob’s syndrome male

  • XYY Males

    • 1 in 1000 live male births

    • extra Y chromosome

    • slightly taller than average

    • more active

    • normal intelligence, slight learning disabilities

    • delayed emotional maturity

    • normal sexual development


Trisomy x
Trisomy X

  • XXX

    • 1 in every 2000 live births

    • produces healthy females

      • Why?

      • Barr bodies

        • all but one X chromosome is inactivated


Turner syndrome
Turner syndrome

  • Monosomy X or X0

    • 1 in every 5000 births

    • varied degree of effects

    • webbed neck

    • short stature

    • sterile


Changes in chromosome structure

error ofreplication

error ofcrossing over

Changes in chromosome structure

  • deletion

    • loss of a chromosomal segment

  • duplication

    • repeat a segment

  • inversion

    • reverses a segment

  • translocation

    • move segment from one chromosome to another


Chromosomal errors vi
Chromosomal errors VI

Deletion

Duplication

Homologouschromosomes

Inversion

Reciprocaltranslocation

Nonhomologouschromosomes


Genomic imprinting
Genomic imprinting

  • Def: a parental effect on gene expression

  • Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.

  • Fragile X syndrome: higher prevalence of disorder and retardation in males


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