AP Biology

March 22, 2012 BellRinger

If two genes are located close together on the same chromosome, how might this affect expected inheritance predictions?

Objective Calculate recombination frequencies Differentiate between various chromosomal

disorders

Homework Chp 14 & 15 quiz

AP Biology© 2011 Pearson Education, Inc.

Animation: Crossing Over Right-click slide / select”Play”

AP Biology

Recombination of Genes: Mendel observed that combinations of traits in

some offspring differ from either parent

Parental types Offspring with a phenotype matching one of the

parental phenotypes are called

Recombinant types/Recombinants Offspring with nonparental phenotypes (new

combinations of traits)

A 50% frequency of recombination is observed for any two genes on different chromosomes

© 2011 Pearson Education, Inc.

AP Biology

Figure 15.UN02

Gametes from green-wrinkled homozygousrecessive parent (yyrr)

Gametes from yellow-rounddihybrid parent (YyRr)

Recombinant offspring

Parental-type

offspring

YR yr Yr yR

yr

YyRr yyrr Yyrr yyRr

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Figure 15.10a

Testcrossparents

Replicationof chromosomes

Gray body, normal wings(F1 dihybrid)

Black body, vestigial wings(double mutant)

Replicationof chromosomes

Meiosis I

Meiosis II

Meiosis I and II

Recombinantchromosomes

Eggs

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vg

b vgb vg

b vg

bvg b vgb vg

b vg

Spermb vg

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Figure 15.10b

Testcrossoffspring

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

Sperm

Parental-type offspring Recombinant offspring

Recombinationfrequency

391 recombinants2,300 total offspring

100 17%

b vg b vgb vgb vg

b vg b vg b vg b vg

b vg

Eggs

Recombinantchromosomes

bvg b vg b vg b vg

AP Biology

As genes are located closer to one another, what happens to their recombination frequency?

Learning Check

AP Biology

If gene A recombines 12% of the time with gene B, but 16% of the time with gene C, which gene is closer to gene A?

If gene B and C recombine with one another 28% of the time, what is the order of genes A, B, and C along the chromosome?

Recombination frequencies

B A C

1612

28

AP Biology

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

Between which two genes would you expect the highest frequency of recombination? A) A and W B) W and E C) E and G D) A and E E) A and G

 

Learning Check

AP Biology 2006-2007

Errors of MeiosisChromosomal Abnormalities

AP Biology

Karyotyping Activty Read through the first page of the

Karyotyping activity

1. What is a karyotype?

2. How are they made?

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Chromosomal abnormalities Incorrect number of chromosomes

nondisjunction chromosomes don’t separate properly

during meiosis breakage of chromosomes

deletion duplication inversion translocation

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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

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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

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

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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

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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

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Down syndrome & age of mother

Mother’s age

Incidence of Down Syndrome

Under 30 <1 in 1000

30 1 in 900

35 1 in 400

36 1 in 300

37 1 in 230

38 1 in 180

39 1 in 135

40 1 in 105

42 1 in 60

44 1 in 35

46 1 in 20

48 1 in 16

49 1 in 12

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Genetic testing Amniocentesis in 2nd trimester

sample of embryo cells stain & photograph chromosomes

Analysis of karyotype

Rate of miscarriage due to amniocentesis: 1970s data

0.5%, or 1 in 200 pregnancies

2006 data<0.1%, or 1 in 1600 pregnancies

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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

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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

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Klinefelter’s syndrome

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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

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Trisomy X XXX

1 in every 2000 live births produces healthy females

Why? Barr bodies

all but one X chromosome is inactivated

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Turner syndrome Monosomy X or X0

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

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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

AP Biology 2006-2007

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