Copyright © 2010 Pearson Education, Inc. Chapter 6 Are You Only As Smart As Your Genes? Mendelian and Quantitative Genetics

Copyright © 2010 Pearson Education, Inc.

Chapter 6

Are You Only As Smart As Your Genes?

Mendelian and Quantitative Genetics


6.1 The Inheritance of Traits

The Inheritance of Traits Offspring resemble their parents, but not

exactly. Siblings resemble each other, but not

exactly. How much is because of environment? How much is inherited?

Nature Versus Nurture

Copyright © 2010 Pearson Education, Inc. Figure 6.1

Adult Gametes Single-celled embryo Multicellular embryo

Sperm

FertilizationZygote

Body axisestablishment,

tissuedifferentiation,organ system

formation

Mitosisand

differentiation

Egg

Mother’s egg and father’ssperm each contain half of the information to “build ahuman.”

This single cell containsall the information on“how to build a human.”

Meiosis


The human life cycle: Adults produce gametes in their gonads by

meiosis. Sperm cells fertilize egg cells to form single-

celled zygotes. Repeated cell divisions form the embryo.



The human life cycle, cont.: The embryo grow to become a fetus. After birth, the individual continues to grow

until reaching adulthood.

Figure 6.1

Fetus

Mitosisand

differentiation

Birth

Mitosisand

differentiation

Mitosisand

differentiation

Baby Child Adult



Genes are segments of DNA that code for proteins. Analogous to words in an instruction manual

for building a human

Figure 6.3

Genesexpressed inmuscle cell

build

build dark brown eyes Genes expressedin eye cell

strong heart muscle

build grow long dark blood brown strong hair for eyes heart small red muscle



Chromosomes are analogous to pages in the instruction manual. Each “page” contains thousands of “words” Different types of cells use different words, in

different orders


6.1 The Inheritance of Traits - Producing Diversity in Offspring

Mistakes in copying DNA (mutations) produce different versions of genes (alleles), with different results.

Figure 6.4

nerve

nzrve

strong

string

grey

gray

Normal allele:

Mutant allele:

Mutation Mutation Mutation

(a) The mutant allele has the samemeaning (mutant allele functionthe same as the original allele).

(b) The mutant allele has a differentmeaning (mutant allele functionsdifferently than the original allele).

(c) The mutant allele has nomeaning (mutant allele isno longer functional).

Original CopyOriginal Copy Original Copy



Parent cell has two complete copies of the manual: 23-page copy from mom and 23-page copy from dad 23 pairs of homologous chromosomes

Figure 6.5

+ =

Egg Sperm Zygote

The 23 pages of each instruction manualare roughly equivalent to the 23chromosomes in each egg and sperm.

The zygote has 46pages, equivalent to 46chromosomes.



Meiosis creates variation in offspring Segregation: in meiosis, one member of

each homologous pair goes into a gamete Gamete gets just one copy of each page of

the manual Independent assortment randomly

determines which member of a pair of chromosomes goes into a gamete Due to random alignment during

metaphase I About 8 million different combinations of

chromosomes.



Siblings share 50% of alleles with each other, on average

Figure 6.6

Possible sperm cell 1 Possible sperm cell 2

Sperm and egg cells each have only 1 full set—a random combinationof maternal and paternal instruction manual pages.

Parent cells have 2 copies of each chromosome—that is, 2 full sets ofinstruction manual pages, 1 from each parent.

Page 3Blood-groupgene from dad

Page 9Eye-color genesfrom mom

Page 3Blood-groupgene from mom

Page 9Eye-colorgenes fromdad



Random fertilization produces more diversity: 64 trillion possibilities!

No two humans are genetically identical, except for monozygotic twins.

Figure 6.7

ZygoteZygote Zygote

EmbryoEmbryo Embryo

Embryosplits

Twoembryos

50% identical (no moresimilar than siblings born at

different times)

100% genetically identical

Egg SpermEgg Sperm Egg Sperm

(a) Dizygotic (fraternal) twins (b) Monozygotic (identical) twins


6.2 Mendelian Genetics: When the Role of Genes Is Clear

Gregor Mendel: first to accurately describe rules of inheritance for simple traits

Controlled mating between pea plants



Gregor Mendel Studied traits due to a

single gene with a few alleles

Discovered that both parents contribute equally to offspring (genetically)

Mendel’s principles also apply to many genetic diseases in humans



Phenotype: physical traits of an individual Genotype: description of the alleles for a

particular gene in an individual Homozygous (-ote): both alleles for a gene

are identical Heterozygous (-ote): the gene has two

different alleles Recessive: the phenotype of an allele is

seen only when homozygous Dominant: the phenotype is seen when

homozygous or heterozygous


6.2 Mendelian Genetics - Genetic Diseases in Humans

Genetic Diseases in Humans Cystic fibrosis: a recessive human

genetic disease Defect in chloride ion transport Causes recurrent lung infections,

dramatically shortened lifespans Heterozygotes (carriers) do not show the

symptoms Most common recessive disease among

Europeans


6.2 Mendelian Genetics - Genetic Diseases in Humans

Huntington’s disease a dominant human genetic disease Progressive, incurable, always fatal Symptoms occur in middle age Mutant protein forms clumps inside nerve

cell nuclei, killing the cells Having a normal allele cannot compensate

for this


6.2 Mendelian Genetics - Using Punnett Squares to Predict Offspring Genotypes

Using Punnett Squares to Predict Offspring Genotypes

Punnett square: graphic way to predict possible outcomes of a cross

Consider a cross between two cystic fibrosis carriers

“F” = normal allele; “f” = recessive disease allele

The cross would be: F f x F f What offspring could result?


6.2 Mendelian Genetics - Using Punnett Squares to Predict Offspring Genotypes

Figure 6.13

FF

Ff

Ff

ff

F

f

25% chance that a child willnot have cystic fibrosis

50% chance that a child willbe an unaffected carrier ofthe cystic fibrosis allele

25% chance that a child willhave cystic fibrosis

Femalecarrier Ff

SpermsampleFf

Ff

Possible types of eggs

Poss

ible

types

of

sperm


6.3 Quantitative Genetics: When Genes and Environment Interact

Qualitative traits are on or off traits

- such as wrinkled or smooth

Quantitative traits show continuous variation Large range of phenotypes E.g., height, weight, intelligence

Variation due to both genetic and environmental differences

Heritability: proportion of the variation within a population due to genetic differences among individuals



Distribution of Phenotypes in Population Mean: sum up all the phenotypic values

and divide by the number of individuals; same as the average.

Figure 6.16aHeight (ft, in)

Mean

Bell-shapedcurve

(a) Normal distribution of student height in onecollege class

5 ft, 10 in (1.78 m )

Variability

Num

ber

of

men



Variance: a measure of how much variability there is in the population The amount an individual varies from the

mean, on average

Figure 6.16b

Weight (lbs)

Mean = 114 lbs(51.7 kg)

(b) Variance describes the variability around themean.

Low variance

High variance

Nu

mber

of

jock

eys

Nu

mber

of

14

-year-

old

boys


6.3 Quantitative Genetics -

Why Traits Are Quantitative Quantitative traits, with continuous

variation, are polygenic traits. Result of several genes Each with more than one allele

Interaction of multiple genes with multiple alleles results in many phenotypes.

Example: human eye color


6.3 Quantitative Genetics

Why Traits Are Quantitative

Usually influenced by both genes and environment

Monozygotic twins, genetically identical, but different environments

Figure 6.17


6.3 Quantitative Genetics –

Measuring Heritability in Animals

Artificial selection: Only the cow giving

the most milk was allowed to breed

The next generation has a higher mean milk production

Milk production has a high heritability



Measuring Heritability in Animals

When artificial selection is impossible, correlations between relatives estimates heritability.

Figure 6.20

StrongAverageWeak

On average,parents andoffspring hadsame level ofimmunity.

Points represent parent-offspringpairs with matching immunity levels.

Blue tit parent immune response

Blu

e t

it c

hic

k im

mune r

esp

onse



Calculating Heritability in Human Populations Have to use correlation to measure heritability in

humans Scientists seek “natural experiments”,

situations in which either the overlap in genes or environment is removed

Twins are often used Dizygotic twins share environment, but only half their

genes Heritability of IQ from such twin studies estimated to

be about 0.52 Similar treatment of twins might explain why their

IQs are so similar



Calculating Heritability in Human Populations

Another approach: Monozygotic twins raised apart share all

genes Estimates of IQ heritability for such twins is

0.72 Drawback: limited number of such twins to

study


6.4 Genes, Environment, and the Individual –

The Use and Misuse of Heritability Differences between groups may be

environmental, despite a high heritability A heritability value pertains just to the population in

which it was measured, and to the environment of that population

Imagine a laboratory population of mice of varying weights Divide this population into 2 genetically identical

groups Give one group a rich diet, the other a poor diet The “rich diet” mice will be bigger than the “poor

diet” mice.


3

2

1 Start with a population ofmice that are variable in size.

Randomly divide miceinto two groups. Feedhalf a poor diet and theother half a rich diet.

Allow the mice in bothgroups to breed.Measure the weight ofadult offspring.

Average weight of the mice in therich- diet environment is twice theaverage weight of the population inthe poor- diet environment.However, there is no geneticdifference between the two groups.


The Use and Misuse of Heritability Allow the mice in each

group to breed, maintaining their diets.

Measure the weight of adult offspring; correlation with parents shows high heritability

Figure 6.22



The Use and Misuse of Heritability Instead of body weight in mice, consider

IQ in humans. Affluent group: higher IQs Impoverished group: lower IQs

Conclude that the difference is probably due to genetics?



The Use and Misuse of Heritability A highly heritable trait can still respond to

environmental change. Maze-learning ability is highly heritable in

rats. Bright rats have bright offspring Dull rats have dull offspring

Still, no rats learned well in a restricted environment. All rats learned better in an enriched

environment



The Use and Misuse of Heritability Heritability does not tell us about individual

differences

Heritability is based on variances in populations, not individuals

High heritability value for a trait does not automatically mean that most of the difference between two individuals is genetic.



How Do Genes Matter? Genes have a strong influence on even

complex traits. But, independent assortment of multiple

genes with multiple alleles produces a large number of phenotypes.

Environment can also have big effects. For quantitative traits, it is difficult to

predict the phenotype of children from the phenotypes of the parents

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Copyright © 2010 Pearson Education, Inc. Chapter 6 Are You Only As Smart As Your Genes? Mendelian and Quantitative Genetics