The following is a map of four genes on a chromosome:

a) Between which two genes would you expect the lowest highest frequency of recombination?

3

Sex-linked Trait ProblemSex-linked Trait Problem

Example: Eye color in fruit flies (red-eyed male) x (white-eyed female)

XRY x XrXr

Remember: the Y chromosome in males does not carry traits.RR = red eyedRr = red eyedrr = white eyedXY = maleXX = female

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4

Sex-linked Trait Solution:Sex-linked Trait Solution:

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female

50% white eyed male

XR

Xr Xr

Y

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5

Female CarriersFemale Carriers

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Pleiotrophy

Example: Sickle cell. A mutation in a gene causes multiple symptoms

Epistasis

In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes.

Polygenic Inheritance

Polygenic inheritance is when a single trait is controlled by 2 or more sets of alleles. Most human traits are polygenically inherited. Examples would be skin and eye color. This explains how you can have several different phenotypes for one trait and how parents can have offspring with eye color or skin color different from what they have.

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) Sex-linkage: genes located on a

sex chromosome Linked genes: genes located on the

same chromosome that tend to be inherited together

autosomal

chromosomes

sexchromosomes

Classes of chromosomes

Huh!Sex matters?!

F2generation

100%red-eye female

50% red-eye male50% white eye male

Discovery of sex linkage

P X

F1generation(hybrids)

100%red eye offspring

true-breeding white-eye male

true-breedingred-eye female

RR rr

What’s up with Morgan’s flies?

x

r r

R

R

Rr

Rr Rr

Rr

100% red eyes

Rr Rr

x

R r

R

r

RR

Rr rr

Rr

3 red : 1 white

Doesn’t workthat way!

Linked genes

http://bcs.whfreeman.com/thelifewire/content/chp10/1002s.swf

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

develop as a female: XX gene redundancy,

like autosomal chromosomes

– an X & Y chromosome develop as a male: XY no redundancy

Genetics of Sex

X Y

X

X

XX

XY

XY

50% female : 50% maleXX

XRXR XrY

Let’s reconsider Morgan’s flies…

x

Xr Y

XR

100% red eyes

XR

XRXr XRY

XRYXRXr

x

XRXr XRY

XR Y

XR

Xr

XRXr

XRYXRXR

XrY100% red females50% red males; 50% white males

BINGO!

Genetic recombination

Crossing over Genes that DO NOT

assort independently of each other

Genetic mapsThe further apart 2

genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency

Linkage mapsGenetic map based on

recombination frequencies

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

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

Hh x HHXHYXHXh

XHXh

XH

Xh

XHY

Y

XH

sex-linked recessive

XH Ymale / sperm

XH

Xh

fem

ale

/ eg

gs XHXH

XHXh

XHY

XhY

XHXH XHY

XHXh XhY

carrier disease

AP Biology

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”

XH

Xh

XHXh

patches of black

patches of orange

tricolor catscan only befemale

Human sex-linkage

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

2006-2007

Errors of MeiosisChromosomal Abnormalities

Chromosomal abnormalities

Incorrect number of chromosomes– nondisjunction

chromosomes don’t separate properly during meiosis

– breakage of chromosomes deletion duplication inversion translocation

AP Biology

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

n

n-1

n+1

AP Biology

Alteration of chromosome number

all with incorrect number 1/2 with incorrect number

error in Meiosis 1

error in Meiosis 2

AP Biologytrisomy

2n+1

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

monosomy2n-1

n-1 n

AP Biology

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

AP Biology

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

AP Biology

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

AP Biology

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

Klinefelter’s syndrome

AP Biology

Klinefelter’s syndrome

AP Biology

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

AP Biology

Trisomy X XXX

1 in every 2000 live births produces healthy females

Why? Barr bodies

all but one X chromosome is inactivated

AP Biology

Turner syndrome Monosomy X or X0

1 in every 5000 births varied degree of effects webbed neck short stature sterile

AP Biology

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

err

or

of

rep

lica

tio

ne

rro

r o

fc

ros

sin

g o

ve

r

Chromosomal errors VI

Deletion

Duplication

Inversion

Reciprocaltranslocation

Nonhomologouschromosomes

Homologouschromosomes

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