Chapter 12 – Patterns of Inheritance

What is inheritance?

What is the relationship between genes, alleles, phenotype and genotype?

How does inheritance of genes influence an organism’s characteristics?

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Understanding genetics requires understanding eukaryotic chromosome structure and the process of meiosis!

What is inheritance? (p. 222)

The genetic transmission of characteristics from parents to offspring

What is the relationship between genes, alleles and genes?

Alleles are differences (variation) in a gene within a population of a species (p. 222)

Figure 12-1 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

The Bk locus contains the Bk gene, (fruit shape).. This plant is heterozygous (p. 222) for the Bk gene (Each chromosome carries a different allele of the Bk gene)

chromosome 1 (pair of homologouschromosomes)from tomato

The M locus (p. 222)contains the M gene (leaf color). This plant is homozygous (p. 222) for the M gene (Both chromosomes carry the same allele of the M gene)

Diploid species possess two alleles for each gene (p. 222)

The two alleles constitute the genotype for a particular gene (p. 225)

What is the relationship between genes, alleles, and genotype?

What is the relationship between genes, alleles, and phenotype?

Several different alleles, each with a different phenotype (p. 225), may exist for a given gene

Table 12-1 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

A single gene can have multiple alleles (p. 233)

Why are there alleles?

Human blood type alleles (p. 235)

Alleles of a gene have nucleotide differences (due to point mutations)

Alleles of genes produce variations of the ‘wild type’ protein

Alleles & the genetics of odor perception

The OR7D4 gene

Hypothesis: Polymorphisms in odorant receptors contribute to variability in human odor perception

Alleles include wild type (typical), and mutant types (super-functioning, partial functioning,

non-functioning)

The genetics of odor perception

RT allele (R88W & T133M impair receptor

function)

WM allele S84N gene variation (allele) - increased

sensitivity to androstenone (1% of population)

Phenotypes (traits) are genetically influenced in different ways*

Types of genetic inheritance

• Single gene – complete dominance

• Single gene – incomplete dominance

• Single gene – codominance

• Single gene – multiple traits (pleiotropy)

• Multiple genes – polygenic inheritance*Environment influences gene expression, therefore environment influences phenotypes

For single gene inheritance, the type of interaction between the pair of inherited

alleles for a gene determines the phenotype

Single gene – complete dominance

Dominant allele – expressed, regardless of the other allele present

Recessive allele – not expressed in the presence of a dominant allele

Rr Rr

Rr Rr rr RR

Genotypes describe the pair of alleles present for a gene (p. 225)

Genotype representations:

Homozygous dominant

AA

Homozygous recessive

aa

Heterozygous

Aa

Gregor Mendel (1822-1884)

Gregor Mendel laid the foundations for understanding inheritance of single gene traits

(p. 223-227)

Observing phenotypic outcomes of specific “crosses” helps to identify mode of

inheritance

First-generationoffspring (F1)

Parentalgeneration (P)

cross-fertilize

Figure 12-4 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Crosses, meiosis & alleles

• Crossing infers sexual reproduction• Sexual reproduction requires meiosis• Meiosis separates pairs of alleles for any given

gene (Mendel’s Law of Segregation, p. 224)

gametesheterozygous parent

A a A a

Pp

P

p

spe

rm

eggsP

p

1/4

1/4

1/4

1/4

1/2

1/2

1/2 1/2

PP Pp

pP pp

Pp

Outcomes of crosses are symbolically represented utilizing Punnett squares (p. 226)

Male parent genotype: Rr

Female parent genotype: Rr

Pp

P

p

spe

rm

eggsP

p

1/4

1/4

1/4

1/4

1/2

1/2

1/2 1/2

PP Pp

pP pp

Pp

Outcomes of crosses are symbolically represented utilizing Punnett squares (p. 226)

Punnett squares predict expected outcomes

Predictions require an assumption about the type of genetic inheritance

The typical assumption is single gene inheritance with complete dominance

If phenotypic ratios of offspring differ significantly than the expected outcome, then it is likely a different type of genetic inheritance is involved

Predicting expected outcomes

Cross Aa x Aa

Expect 2 phenotypes in a 3:1 ratio (assuming single gene/complete dominance)

Outcome of cross: 3 phenotypes in a 2:1:1 ratio

Does inheritance of this trait involve a single gene with complete dominance?

Not all genes/alleles display dominant/recessive interactions

(p. 233-235)

Codominance – both alleles for a gene are expressed (neither functions as a recessive allele)

Incomplete dominance – heterozygous phenotype is different from either the dominant or recessive phenotype

Codominance = heterozygote expresses both alleles (p. 235)

A and B are dominant to O (i); A & B are codominant

Figure 12-24 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

spe

rm

eggs

mother

father

C2C1

C2C1

C2C1 C2

C1

C1C1 C1C2

C1C2 C2C2

Human hair texture alleles exhibit incomplete dominance (p. 233)

A few disease-causing alleles function as dominant alleles in

single gene inheritance (p. 239)

Heterozygous persons have the disease/condition

Forms of deafnessHuntington’s diseaseMyotonic dystrophy

Figure 12-25b Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Many traits are influenced by more than one gene (p. 235-236)

Figure 12-25a Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

sperm

eggs

Polygenic inheritance of human skin color involves 3 dinstinct genes (p. 236)

Sex-linked traits are encoded by genes on the X or Y chromosome (p. 231-233)

• Genes located on X-chromosome

• Males possess only 1 allele for sex-linked traits

-In males, recessive disease alleles cannot be masked by a dominant WT allele

Sex-linked trait example

Red eyes is dominant to white eyes in fruit flies. A homozygous dominant female is crossed to a white eyed male. What will be the predicted phenotypic frequencies in the offspring?

Figure 12-27 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

(a) A pedigree for a dominant trait

I

III

II

I

III

II

IV

? ? ? ?

? ? ? ? ?

How to read pedigrees

or

or

or

or

= cannot determine genotypefrom pedigree

= known carrier (heterozygote)for recessive trait

= does not show trait

= shows trait

= offspring

= parents

= female= male

I, II, III = generations

(b) A pedigree for a recessive trait

Geneticists use pedigrees to understand inheritance of human characteristics (p. 237)

Figure 12-30b Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

II

maternalgrandfather

= normal color vision (not carrier)

= heterozygous carrier female,normal color vision

= colorblindor

or

daughter

G. Audesirk T. Audesirksister

aunts fathermother

? ?IV

III

I

Sex-linked pedigree (p. 240)

Analyzing pedigrees

Are both genders affected by the disorder?

Sex-linked disorders typically affect only males

How frequent do affected individuals occur?

Infrequent suggests the disorder is recessive to the wild type allele

Genetics and environment (p. 237)

Some characteristics are fully genetic (example: human eye color)

Many characteristics are influenced by both genes and environment (example: hydrangea flower color and soil pH)

The same allele in different environments can result in different outcomes

Different alleles in the same environment can result in different outcomes

Role of Genotype in the Cycle of Violence in Maltreated children

Role of Genotype in the Cycle of Violence in Maltreated children

What are the social & legal implications of this research?

If a violent criminal offender (with a known family history of maltreatment) is shown to have the low expression MAOA allele by a genetic test, can/should he be held responsible for his violent criminal behavior?

What is genetic linkage? (p. 229)

Genes located close together on the same chromosome are genetically linked (i.e they tend to get inherited together)

Linkage between genes varies with distance between genes

A B C D

a b c d

Tightly linked genes

Some genes are so tightly linked that they are rarely/never separated by crossing over

Observing phenotypic ratios of offspring produced in a dihybrid cross allows one to determine of linkage or independent assortment is occurring between two distinct genes

a

AB

b a

A

b

BIndividual with genotype AaBb

Gametes, assuming complete linkage

a

AB

b

AaBb

a b

A BAB

ab

Complete linkage

AaBb x AaBb

AB

AB

ab

ab

AABB

AaBb

AaBb

aabbaa

A B

bb

What is the phenotypic ratio?

Gametes, assuming independent assortment

a

A

b

B

a

A

B

b

a

A B

b

or

Ab

aB

AB

ab

AaBb 25%

25%

25%

25%

Two genes exhibiting independent assortment (p. 228)

AABB

AABb

aabb

AB

Ab

aB

ab

AB Ab aB ab

Genetic linkage can be broken by crossing over (p. 230)

Genotype:

AaBbGametes

produced:

AB

Ab

aB

ab

Working genetics problems

• Phenotypic ratios of offspring vary and depend on the genotypes of the parents and the type of inheritance (3:1 ratio in monohybrid crosses and 9:3:3:1 ratio in dihybrid crosses only apply to single gene/complete dominance traits)

• Practice problems: p. 246-248

Red flowers and thorns are dominant to white flowers and no thorns in

roses. Which of the following can be a genotype for a red, thorny rose?

1) RT

2) RRTT

3) RrTT

4) RrTt

5) 2, 3 & 4

Red flowers and thorns are dominant to white flowers and no thorns in roses. What

would be the phenotype of a rose heterozygous for both traits?

1) RrTt

2) Red, thorny

3) RRTT

4) White, thornless

5) rrtt

Which of the following represents the genotype of a gamete produced by an individual of genotype Aabb (assuming

independent assortment)?

1. Aa

2. Bb

3. AB

4. Ab

5. Aabb

6. Both 1 and 2

7. Both 3 and 4

8.None of these

Assume flower color in daffodil is determined by a single gene. A yellow flowered daffodil is crossed

with a white flowered daffodil. All 100 of the offspring produce yellow flowers. What can you

conclude?1. The yellow flowered parent is homozygous for

the flower color gene2. The white flowered plant is homozygous for the

flower color gene3. The yellow allele is dominant to the white allele4. All of the above5. Both 2 and 3

Which genes are most likely to be inherited together?

1) 2 & 2a

2) 1 & 5

3) 1a&2a

4) 5&7

5) 1a & 4a

6) Responses 3, 4 & 5 are all correct

1 2 3 4

56 7

1a 2a 3a 4a

5a 6a 7a