Ch. 15 Inheritance 9e

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Ch. 15 Warm-Up

1. A white-eyed female fruit-fly is mated with a

red-eyed male. What genotypes and phenotypes do you predict for the offspring?

1. Neither Tim nor Rhoda has Duchenne muscular dystrophy (X-linked recessive disorder), but their firstborn son has it. What is the probability their 2nd

child will have it?

1. Colorblindness is a sex-linked recessive trait. A colorblind male and a female with normal vision have a son who is colorblind. What are the parents’ genotypes?

Definitions:

• Sex-linked gene

• Barr body • SRY gene • Linked

genes • Linkage

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Warm up

1. What is a Barr body?

2. How are linkage maps constructed? (See. Fig. 15.11 in Campbell 9th ed.)

3. Determine the sequence of genes along a

chromosome based on the following recombination frequencies: B, 8 map units; C, 19 map units; A-D, 20 map units; B-C, 11 map units; B-A-D, 28 map

units.

4. What does a frequency of recombination of 50% indicate?

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1. What is the pattern of inheritance of the trait (shaded square/circle) shown in the pedigree?

1. How many chromosomes are in a human cell that is:

a) Diploid? b) Triploid?

c) Monosomic? d) Trisomic?

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THE CHROMOSOMAL BASIS OF

INHERITANCE

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What you must know:

• _{How the chromosome theory of inheritance}

connects the physical movement of chromosomes

in meiosis to Mendel’s laws of inheritance.

• _{The unique pattern of inheritance in sex-linked}

genes.

• _{How alteration of chromosome number or}

structurally altered chromosomes (deletions,

duplications, etc.) can cause genetic disorders.

• _{How genetic imprinting and inheritance of}

mitochondrial DNA are exceptions to standard

Mendelian inheritance.

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Chromosome theory of inheritance

:

• _{Genes have specific}

locations (loci) on chromosomes

• _Chromosomes

segregate and assort independently

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Thomas Hunt Morgan

• _{Drosophila melanogaster}

_{– fruit fly}

– _{Fast breeding, 4 prs. chromosomes (XX/XY)}

• _{Sex-linked gene}

_{: located on X or Y}

chromosome

– _{Red-eyes = wild-type; white-eyes = mutant} – _{Specific gene carried on specific chromosome}

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Sex determination

varies between

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Sex-linked genes

• _{Sex-linked gene on X or Y}

• _{Females (XX), male (XY)}

–

_{Eggs = X, sperm = X or Y}

• _{Fathers pass X-linked genes to daughters, but not}

sons

• _{Males express recessive trait on the single X}

(

hemizygous

)

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Sex-linked disorders

• _{Colorblindness}

• _{Duchenne muscular dystrophy}

• _Hemophilia

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X-Inactivation

Barr body = inactive X chromosome; regulate gene dosage in females during embryonic development

• Cats: allele for fur color is on X

• Only female cats can be tortoiseshell or

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Human development

• _{Y chromosome required for development of testes} • _{Embryo gonads indifferent at 2 months}

• _{SRY gene}_{: sex-determining region of Y}

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Genetic Recombination

: production of offspring

with new combo of genes from parents

• _{If offspring look like parents}



parental types

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• If results do not follow Mendel’s Law of Independent Assortment, then the genes are probably linked

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Linked genes

: located on same chromosome and

tend to be inherited together during cell division

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Crossing over

: explains why some linked genes

get separated during meiosis

• _the

_{further apart 2 genes on}

_further

_{same chromosome}

_,

the

higher the probability of

higher

crossing over

and

the

higher the

higher

recombination frequency

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Linkage Map

: genetic map that is based on

% of cross-over events

• ₁_{map unit}_{= 1% recombination frequency} • _{Express relative distances along chromosome}

• _{50% recombination}_{= far apart on}_same_{chromosome or}

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Exceptions to Mendelian

Inheritance

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Genomic Imprinting

• _{Genomic imprinting}_{: phenotypic effect of gene} depends on whether from M or F parent

• _Methylation_{: silence genes by adding methyl groups} to DNA

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Non-Nuclear DNA

• _{Some genes located in}

organelles

– _{Mitochondria, chloroplasts,}

plastids

– _{Contain small circular DNA} • _{Mitochondria = maternal}

inheritance (eggs)

Variegated (striped or spotted) leaves result from mutations in pigment genes in plastids, which generally are inherited from the maternal parent.

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Genetic Testing

Reasons for Genetic Tests:

• _{Diagnostic testing (genetic disorders)}

• _{Presymptomatic & predictive testing}

• _{Carrier testing (before having children)}

• _{Pharmacogenetics (medication & dosage)}

• _{Prenatal testing}

• _{Newborn screening}

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Prenatal Testing

• _{May be used on a fetus to detect genetic}

disorders

• _{Amniocentesis}

_{: remove amniotic fluid}

around fetus to culture for karyotype

• _{Chorionic villus sampling}

_{: insert narrow tube}

in cervix to extract sample of placenta with

fetal cells for karyotype

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Nondisjunction

: chromosomes fail to separate

properly in Meiosis I or Meiosis II

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Karyotyping

can detect nondisjunctions.

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Klinefelter Syndrome: 47XYY, 47XXY

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Nondisjunction

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Nondisjunction

• Aneuploidy: incorrect # chromosomes

– Monosomy (1 copy) or Trisomy (3 copies)

• Polyploidy: 2+ complete sets of chromosomes; 3n or 4n

– Rare in animals, frequent in plants

A tetraploid mammal. Scientists think this species may have arisen when an ancestor doubled its chromosome # by errors in mitosis or meiosis.

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1. What is the pattern of inheritance of the trait (shaded square/circle) shown in the pedigree?

1. How many chromosomes are in a human cell that is:

a) Diploid? b) Triploid?

c) Monosomic? d) Trisomic?

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Chi-Square Analysis Practice

• _{Two true-breeding Drosophila are crossed: a normal-winged,}

red-eyed female and a miniature-winged, vermillion-eyed male. The F1 offspring all have normal wings and red eyes.

When the F1 offspring are crossed with miniature-winged,

vermillion-eyed flies, the following offspring resulted:

– _{233 normal wing, red eye}

– _{247 miniature wing, vermillion eye} – _{7 normal wing, vermillion eye}

– _{13 miniature wing, red eye}

• _{What type of conclusions can you draw from this experiment?}