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Chapter 23: Patterns of Gene Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Mendel’s Laws

Gregor Mendel was an Austrian monk who in 1860 developed certain laws of heredity after doing crosses between garden pea plants.

Gregor Mendel investigated genetics at the organismal level.

Examples of traits that can be observed at the organismal level include facial features (ex: big noses) that cause generations to resemble each other.

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Gregor MendelMendel’s law of segregation:

1.) Each individual has two factors (called genes) for each trait (one from each parent).

2.) The genes segregate (separate) during gamete formation (i.e., meiosis).

3.) Each gamete contains only one gene for each trait (i.e., they are haploid).

4.) Fertilization gives the new individual two genes for each trait (one from each parent, restores diploid state).

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Homologous Chromosomes

Genes

From Father From Mother

Diploid = Two copies of each type of chromosome

Loci = Physical position of a gene on a chromosome

Allele = Alternate forms of a gene:

Alleles have the same position (locus) on a pair of homologous chromosomes

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Alleles code for the same trait.

Examples of alleles:-curly or straight (alleles), hair type (gene)-attached or unattached (alleles), ear lobe type (gene)

Chromosomes segregate during the formation of the gametes and each gamete has only one chromosome from each pair.

Fertilization gives each new individual two chromosomes again.

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The Inheritance and Expression of a Single Trait

A capital letter indicates a dominant allele, which is expressed when present.

An example is W for widow’s peak.

A lowercase letter indicates a recessive allele, which is only expressed only in the absence of a dominant allele.

An example is w for a continuous or straight hairline.

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Widow’s peakWIDOW’S PEAK

WWWw

STRAIGHT or CONTINUOUS HAIRLINEww

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Genotype and Phenotype

Genotype refers to the genes of an individual which can be represented by two letters or by a short descriptive phrase.

Homozygous means that both alleles are the same; for example, WW stands for homozygous dominant and ww stands for homozygous recessive.

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Heterozygous means that the members of the allelic pair are different—for example, Ww.

Phenotype refers to the physical or observable characteristics of the individual – widow’s peak or straight hairline.

Both WW and Ww result in widow’s peak, two genotypes with the same phenotype.

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Gamete FormationBecause homologous pairs separate during

meiosis, a gamete has only one allele from each pair of alleles (for a specific gene).

If the allelic pair is Ww, the resulting gametes would contain either a W or a w, but not both – (gametes are haploid).

Ww represents the genotype of an individual.Gametes that could be produced by this

individual are W or w.

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Genotype

AA

Aa

AABb

AaBb

Gametes

A A

A a

AB Ab

AB Ab aB ab

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One-Trait CrossesIn one-trait crosses, only one trait (such

as type of hairline) is being considered.

When performing crosses, the original parents are called the parental generation, or the P generation.

All of their children are the filial generation, or F generation.

Children are monohybrids when they are heterozygous for one pair of alleles.

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Male Female

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If you know the genotype of the parents, it is possible to determine the gametes and use a Punnett square to determine the phenotypic ratio among the offspring.

W w

w

W WW Ww

Ww ww

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Monohybrid crossGenotypes of parents are known

(both are heterozygous Ww)

Genotypic Ratio1 WW homozygous dominant2 Ww heterzygous3 ww homozygous recessive

Phenotypic Ratio3 widow’s peak1 straight hairline

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The One-Trait TestcrossIt is not always possible to discern a

homozygous dominant from a heterozygous individual by inspection of phenotype (they have the same phenotype – both will have widow’s peak).

A testcross crosses the dominant phenotype with the recessive phenotype.

If a homozygous recessive phenotype is among the offspring, the parent must be heterozygous.

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One-trait testcross

All offspring have dominantphenotype. Therefore the dominant parent (genotype we are tying to figure out) must be homozygous dominant.

?

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?Offspring have dominant andrecessive phenotypes. Therefore the dominant parent (genotype we are tying to figure out) must be heterozygous dominant.

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1.) Both a man and woman are heterozygous for tongue rolling. Tongue rolling is dominant over non-tongue rolling. What is the chance that their child will be a tongue roller?

Male Female

Tt Tt

T t T t

T

t

T t

TT

Tt

Tt

tt

GENOTYPE

GAMETES

MALE

FEMALE

Offspring Phenotypes3 Rollers1 Non-Roller

3 of 4 chances for rollerchild (75% chance).

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2) Both you and your sibling are non-rollers and your parents are rollers. Tongue rolling is dominant over non-tongue rolling. What are the genotypes of your parents?

OFFSPRING tt

PARENTS T t

T

t

T t

TT

Tt

Tt

tt

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The Inheritance of Many Traits

Independent AssortmentThe law of independent assortment states that

each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the gametes.

This law is dependent on the random arrangement of homologous pairs at metaphase.

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Segregation and independent assortment

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Two-Trait CrossesIn two-trait crosses, genotypes of the

parents require four letters because there two alleles for each trait.

Gametes will contain one letter for each trait.

When a dihybrid (heterozygous for both traits) reproduces with another dihybrid the phenotypic results are 9 : 3 : 3 : 1.

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Widow’s Peak is dominant over Straight Hairline W w

Short Fingers are dominant over Long Fingers S s

Phenotype Genotypes

Widow’s Peak / Short Fingers WWSS WWSs WwSS WsSs

Widow’s Peak / Long Fingers WWss Wwss

Straight HL / Short Fingers wwSS wwSs

Straight HL / Long Fingers wwss

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Dihybrid cross (two traits)Widow’s PeakShort Fingers

Straight HairlineLong Fingers

Widow’s PeakShort Fingers

homozygous dominant homozygous recessive

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WwSs

WwSs

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The Two-Trait TestcrossA testcross is done to determine

genotype of individual that has dominant phenotypes (for both traits).

(Homozygous dominant or heterozygous for the two traits under consideration).

Cross heterozygote for both traits with homozygous recessive for both traits - results in 1 : 1 : 1 : 1 ratio.

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Two-trait testcross

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Dominant Recessive

normal skin pigmentation albinismfreckles no frecklesbroad lips thin lipstongue roller non-tongue rollerPTC taster PTC non-tasterlarge eyes small eyesmigraine headaches no migraine headachesnormal foot arch flat feet

SELECTED TRAITS IN HUMAN HEREDITY

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If a man that is homozygous recessive for eye size (i.e., has small eyes) and is homozygous dominant forfreckles (i.e., has freckles) has children with a womanthat is homozygous dominant for eye size (i.e., has large eyes) and is homozygous recessive for freckles (i.e., does not have freckles), what are the potential phenotypes and genotypes of their children?

Man llFF IF only for gametes

Woman LLff Lf only for gametes

ILFf All are heterozygous for both traits and show large eyes with freckles

GENOTYPE PHENOTYPEIF IF

Lf

Lf ILFfILFf

ILFf

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If one of the children reproduces with another person that has the same genotype, what are the chances that they will have a child with large eyes and freckles?

LlFf X LlFf

LF

Lf

lF

lf

LF Lf lF lf

LLFF

LLFf

LlFF

LlFf

LLFf

LLff

LlFf

llffLlff llFf

llFF llFf

LlFf Llff

LlFF LlFf

large eyes/freckles

Large eyes/no freckles

Small eyes/freckles

Small eyes/no freckles

1/16

3/16

3/16

9/16 or 56 %

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Genetic Disorders

Patterns of Inheritance

When studying human disorders, biologists often construct pedigree charts to show the pattern of inheritance of a characteristic within a family.

Genetic counselors construct pedigree charts to determine the mode (dominant or recessive) of inheritance of a condition.

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Pedigree Analysis: determine how a genetic disorderis inherited, chances of offspring having a genetic disorder.

Unaffected male Unaffected female

Affected Male Affected female

“UNION”

OFFSPRING

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Genetic Disorders: medical conditions caused by alleles inherited from parents, hereditary disorder.

Autosomal Genetic Disorders: genetic disorders caused byAlleles on autosomal chromosomes (non-sex Chromosomes – similar to somatic).

Autosomal Disorders can be:1) Autosomal Dominant2) Autosomal Recessive

Autosomal Dominant AA or Aa have disorder (phenotype)

aa

Aa Aa Aa

aa

aa

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Autosomal Recessive: aa have disorder (phenotype)

aa

aa

aa aa

Aa

AA or Aa

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Autosomal recessive pedigree chart (Tay-sachs disease, Cystic fibrosis, PKU)

CARRIER – Has allele butis unaffected

* H

OW

DO

YO

U K

NO

W I

ND

IVID

UA

L I

S H

ET

ER

OZ

YG

OU

S?

*

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Autosomal dominant pedigree chart (Neurofibromatosis, Huntington disease)*

HO

W D

O Y

OU

KN

OW

IN

DIV

IDU

AL

IS

HE

TE

RO

ZY

GO

US

? *

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Polygenic Inheritance

Polygenic traits are governed by more than one gene pair (e.g., several pairs of genes may be involved in determining the phenotype).

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Polygenic inheritance

Such traits produce a continuous variation representing a bell-shaped curve (Ex: height in humans).

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Skin Color

The inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of polygenic inheritance.

A range of phenotypes exist from very dark to very light.

The distribution of these phenotypes also follows a bell-shaped curve.

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Polygenic Disorders

Many human traits, like allergies, schizophrenia, hypertension, diabetes, cancers, and cleft lip, appear to be due to the combined action of many genes plus environmental influences.

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Multiple Allelic Traits

Inheritance by multiple alleles occurs when more than two alternative alleles exist for a particular gene locus.

A person’s blood type is an example of a trait determined by multiple alleles (A, B, and O).

***Each individual inherits only two alleles for these genes.

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ABO Blood TypesA person can have an allele for an A

antigen (blood type A) or a B antigen (blood type B), both A and B antigens (blood type AB), or no antigen (blood type O) on the red blood cells.

Human blood types can be type A (IAIA or IA i), type B (IBIB or IBi), type AB (IAIB), or type 0 (ii).

Alleles: A, B, O

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Inheritance of blood type….(Who’s your daddy?)

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Incompletely Dominant TraitsCodominance means that both alleles are

equally expressed in a heterozygote. (Ex: sickle cell anemia)

Incomplete dominance is exhibited when the heterozygote doesn’t show the dominant trait but shows an intermediate phenotype, representing a blending of traits. (Ex: curly, wavy, or straight hair)

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Incomplete dominance

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Sickle-Cell DiseaseSickle-cell disease is an example of a

human disorder controlled by incompletely dominant alleles.

Sickle cell disease involves irregular, sickle shaped red blood cells caused by abnormal hemoglobin.

HbA represents normal hemoglobin; and HbS represents the sickled condition.

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HbAHbA individuals are normal; HbSHbS individuals have sickle-cell disease and HbAHbS individuals have the intermediate condition called sickle-cell trait.

Heterozygotes have an advantage in malaria-infested Africa because the pathogen for malaria cannot exist in their blood cells.

This evolutionary selection accounts for the prevalence of the allele among African Americans.