Chp 10 Patterns of Inheritance

Transcription

1 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?

2 Wolf Ancestral canine Chinese Shar-Pei Akita Siberian Husky Basenji Alaskan Malamute Afghan hound Saluki Rottweiler Sheepdog Evolutionary relationships: based on DNA similarities and differences Retriever 2

3 Chromosomes Are Packets of Genetic Information What we inherit comes from two parents

4 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 science/ /eric-lander.html 4

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6 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 6

7 Inheritance of a Single Character 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 F 1 gen. all w/ purple flowers F 2 gen. - ¾ purple & ¼ white flowers Crosses tracking one character (flower color) 7

8 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 8

9 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 9

10 What are the genotypes of these plants? What are their phenotypes? What does the Punnet square at the bottom of the diagram show you? Explanation of the crosses in Figure 9.3A. 10

11 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 PARENTS OFFSPRING Eggs d Normal Dd Dd Normal (carrier) Punnet square showing offspring produced by parents who are both carriers for a recessive disorder D DD Normal dd Deaf Normal Dd D Dd Normal (carrier) Sperm d 11

12 Homologous Chromosomes Have Alleles Every individual has two alleles for each gene one from each parent Three gene loci on homologous chromosomes 12

13 Mendel s Laws Apply to Humans 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 13

14 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

15 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

16 Family Pedigrees Track Genetic Traits A pedigree shows inheritance of a trait through multiple generations demonstrates dominant or recessive inheritance can be used to deduce genotypes of family members Examples of single-gene inherited traits in humans 16

17 Figure 9.8B Pedigree showing inheritance of attached versus free earlobe in a hypothetical family. 17

18 Many Inherited Disorders are Single Genes 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 18

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20 Many Genes Have More Than 2 Alleles 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 20

21 In Codominance More Than One Allele Encodes a Protein The I gene has 3 possible alleles: I A, I B and i

22 Blood Group (Phenotype) Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB O Anti-A Anti-B A Anti-B B Anti-A AB 22

23 The Environment Can Alter Phenotype 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. 23

24 Some Traits Depend on Multiple Genes Skin color is a polygenic trait; it is affected by more than one gene. Section 10.9 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 10.23

25 Sex-linked Genes Have Unique Inheritance 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 25

26 X-linked Recessive Disorders Affect Mostly Males WHY? Males only need one recessive allele to be affected Female must get a recessive allele on both X chromosomes to be affected The number 7 The number 96 26

27 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? 27

28 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

29 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

30 Investigating Life: Heredity and the Hungry Hordes 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. Section Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 10.24

31 Investigating Life: Heredity and the Hungry Hordes In a mating between two Bt-resistant bollworms, all of the offspring will also be resistant. Section Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 10.24

32 However, if a resistant bollworm mates with a susceptible bollworm, only some and sometimes none of the offspring will be resistant. Section Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 10.24

33 To avoid 100% resistance among bollworms of future generations, farmers must plant some crops without the toxin gene. Crops with the Bt toxin Section Crops without the Bt toxin Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 10.24

34 This arrangement increases the chance that some susceptible bollworms will remain in the population. Resistance is conferred by a recessive allele Crops with the Bt toxin Section Crops without the Bt toxin Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 10.24

35 10.10 Mastering Concepts Explain the logic of planting non- Bt crop buffer strips around fields planted with Bt crops PhotoDisc, Inc./Getty Images/RF Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.