Chp 10 Patterns of Inheritance

✶ Dogs, one of human’s longest genetic experiments Over 1,000’s of years, humans

have chosen and mated dogs with specific traits. A process called -artificial selection

The result – diverse breeds w/ distinct body types and behaviors

✶ Sequencing dog’s complete DNA - genome - reveals evolutionary relationships bt. breeds What breed of dog is this?

Ancestral canine

Chinese Shar-Pei

Akita

Basenji

Siberian Husky

Alaskan Malamute

Rottweiler

Sheepdog

Retriever

Afghan hound

Saluki

Wolf

Evolutionary relationships: based on DNA similarities and differences

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Chromosomes Are Packets of Genetic Information

What we inherit comes from two parents

Mendel Uncovered Basic Laws of Inheritance

!  Genetics = scientific study of heredity !  Heredity = transmission of traits from one generation

to the next

✷ Gregor Mendel –  Began field of genetics in 1860s, –  Deduced principles of genetics

by breeding garden peas –  Background in math, physics,

and chemistry

✷  Eric Lander: The Human Genome Project http://www.nytimes.com/video/2012/01/02/science/100000001255558/eric-lander.html 4

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Mendel determined •  Parents pass on to offspring discrete units

“heritable factors” •  Heritable factors (today called genes), retain

their individuality generation after generation

Genetic myths

– Characteristics acquired in life can be passed on

– Characteristics of parents blend irreversibly in offspring

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✶  Mendel performed a monohybrid cross

–  Cross bt. 2 indivs differing in a single character

✶  Crossed plant with purple flowers and plant with white flowers •  The F1 gen. – all w/

purple flowers •  F2 gen. - ¾ purple & ¼ white flowers

Inheritance of a Single Character

Crosses tracking one character (flower color)

Mendel developed 4 hypotheses - (modern terms used below)

1. Alleles are alternative versions of genes

2. An organism inherits 2 alleles, one from each parent. They can be the same, or different

A homozygous genotype has identical alleles A heterozygous genotype has two different

alleles

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3. If alleles differ, then one determines organism’s appearance = dominant. The other has no noticeable effect on organism’s appearance = recessive -  The phenotype is the appearance or expression

of a trait -  The genotype is the genetic makeup of a trait

-  The same phenotype may be determined by more than one genotype

4.  A sperm or egg carries only 1 allele, b/c allele pairs separate (segregate) from each other during the production of gametes. This is called the law of segregation

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Explanation of the crosses in Figure 9.3A.

What are the genotypes of these plants?

What are their phenotypes?

What does the Punnet square at the bottom of the diagram show you?

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Punnet square showing offspring produced by parents who are both carriers for a recessive disorder

D D

d d

Normal Dd

Normal Dd

DD Normal

Dd Normal (carrier)

Dd Normal (carrier)

dd Deaf

Eggs Sperm

PARENTS

OFFSPRING

Genotype, Phenotype and the Punnet Square

✶ Genotype –  The genetic

makeup shown by pair of alleles

–  Ex: AA, Aa, or aa

✶ Phenotype -  The observable,

physical expression of genotype

-  Ex: flower color, eye color, deafness

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✶ Every individual has two alleles for each gene – one from each parent

Homologous Chromosomes Have Alleles

Three gene loci on homologous chromosomes

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✶ Segregation and fertilization are chance events

✶ Each allele combination has a given probability of occurring

✶ Punnet square shows all possible combinations of ______ in offspring

Segregation and fertilization as chance events

Mendel’s Laws Apply to Humans

Cystic Fibrosis Example

Lets say: Two healthy parents visit a genetic counselor

✶ Counselor tells them: Both of you are heterozygous for Cystic Fibrosis

✶ What is the probability of them having a child with Cystic Fibrosis?

✶ CF manifests itself in the homozygous recessive individual

Question

Cystic fibrosis is caused by a recessive allele. If a healthy carrier and an affected individual have a child, what is the chance the child will be affected?

A.  1/4 B.  1/3 C.  1/2 D.  3/4 E.  1

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✶ A pedigree – shows inheritance of a trait

through multiple generations

– demonstrates dominant or recessive inheritance

– can be used to deduce genotypes of family members

Family Pedigrees Track Genetic Traits

Examples of single-gene inherited traits in humans

Figure 9.8B Pedigree showing inheritance of attached versus free earlobe in a hypothetical family.

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✶  Inherited human disorders

–  Recessive inheritance

•  2 recessive alleles needed to show disease

•  Heterozygous parents are carriers

•  Probability of inheritance increases w/ inbreeding

–  Dominant inheritance

•  1 dominant allele is needed to show disease

•  Dominant lethal alleles usu. eliminated from a population

Many Inherited Disorders are Single Genes

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✶ Multiple alleles

–  More than 2 alleles found in the population

–  A diploid individual can only carry _____ of these alleles

–  Ex: ABO blood group has 3 alleles, leading to 4 phenotypes; type A, type B, type AB, and type O blood

✶ Codominance –  Neither allele is dominant

–  Expression of both alleles is a distinct phenotype

–  Ex. type AB blood

Many Genes Have More Than 2 Alleles

In Codominance More Than One Allele Encodes a Protein

The I gene has 3 possible alleles: IA, IB and i

Antibodies Present in Blood

Anti-A Anti-B

Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left

Anti-B

O A B AB

—

Anti-A

Blood Group (Phenotype)

O

A

B

AB

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✶ Many phenotypic variations result from combination of genes and environment

–  Nutrition during development (height, weight)

–  Skin color is affected by ________

–  Exposure to drugs, toxins or pollutants and risk of cancer

✶  For example, the enzyme responsible for pigment production in Siamese cat fur is active only in cool body parts.

The Environment Can Alter Phenotype

Some Traits Depend on Multiple Genes

Section 10.9

Skin color is a polygenic trait; it is affected by more than one gene.

Figure 10.23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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✶ Sex-linked genes = genes located on a sex chromosome

✶ Which gamete, the sperm or the egg, determines the sex of the offspring?

✶ Examples: Color blindness, Hemophilia

Sex-linked Genes Have Unique Inheritance

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WHY? ✶ Males only need one recessive allele to be affected ✶  Female must get a recessive allele on both X

chromosomes to be affected

X-linked Recessive Disorders Affect Mostly Males

The number 7 The number 96

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Punnet Square: X-linked Recessive Problem

If mom is a carrier of an X-linked disorder and dad is normal, what are possible outcomes for

- a son? - a daughter?

Question Hemophilia is a X-linked recessive disorder. If an affected female and an unaffected male have a boy, what is the chance he will have hemophilia?

A.  0 B.  1/4 C.  1/2 D.  3/4 E.  1

Mastering Concepts

1.  Distinguish between dominant and recessive; heterozygous and homozygous; phenotype and genotype.

2.  Explain the meaning of: locus, multiple alleles, pedigree, and codominance.

3.  Describe the pattern of inheritance for regular autosomal genes and sex-linked genes.

4.  Solve genetics problem, using punnet squares involving monohybrid crosses for autosomal and sex-linked traits

Investigating Life: Heredity and the Hungry Hordes

Section 10.10

Bollworm larvae devastate cotton crops. But some bollworms are susceptible to Bt toxin. Biologists have inserted the gene encoding this toxin into the cotton genome.

Figure 10.24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Investigating Life: Heredity and the Hungry Hordes

Section 10.10

In a mating between two Bt-resistant bollworms, all of the offspring will also be resistant.

Figure 10.24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Section 10.10

However, if a resistant bollworm mates with a susceptible bollworm, only some—and sometimes none—of the offspring will be resistant.

Figure 10.24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Section 10.10

To avoid 100% resistance among bollworms of future generations, farmers must plant some crops without the toxin gene.

Figure 10.24

Crops with the Bt toxin

Crops without the Bt toxin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Section 10.10

This arrangement increases the chance that some susceptible bollworms will remain in the population.

Figure 10.24

Crops with the Bt toxin

Crops without the Bt toxin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Resistance is conferred by a recessive allele

10.10 Mastering Concepts

Explain the logic of planting non-Bt crop buffer strips around fields planted with Bt crops.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © 1996 PhotoDisc, Inc./Getty Images/RF