The Inheritance of Traits Offspring resemble their parents

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? © 2013 Pearson Education, Inc.

The Inheritance of Traits § 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. © 2013 Pearson Education, Inc.

§ The human life cycle, cont. : § The embryo grow to become a

The Inheritance of Traits § Genes are segments of DNA that code for proteins. § Analogous to words in an instruction manual for building a human § We have many genes on each of our chromosomes. § 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 © 2013 Pearson Education, Inc.

The Inheritance of Traits - Producing Diversity in Offspring § Mistakes in copying DNA

The Inheritance of Traits - Producing Diversity in Offspring § 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 © 2013 Pearson Education, Inc.

§ 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 § This is due to random alignment during metaphase I § About 8 million different combinations of chromosomes. © 2013 Pearson Education, Inc.

Parent cells have 2 copies of each chromosome—that is, 2 full sets of instruction manual pages, 1 from each parent. §Siblings share 50% of alleles with each other, on average Sperm and egg cells each have only 1 full set—a random combination of maternal and paternal instruction manual pages. Possible sperm cell 1 Page 3 Blood-group gene from dad © 2013 Pearson Education, Inc. Page 9 Eye-color genes from mom Possible sperm cell 2 Page 3 Blood-group gene from mom Page 9 Eye-color genes from dad Figure 6. 6

§ Random fertilization produces more diversity: 64 trillion possibilities! § No two humans are genetically identical, except for monozygotic twins. § Dizygotic twins are 50% identical - just like siblings born at different times. © 2013 Pearson Education, Inc.

Mendelian Genetics: When the Role of Genes Is Clear § Gregor Mendel: first to accurately describe rules of inheritance for simple traits § His research involved controlled mating between pea plants. § His pattern of inheritance occurs primarily in traits that are due to a single gene with a few alleles. § Mendel’s principles also apply to many genetic diseases in humans. © 2013 Pearson Education, Inc.

§ 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 © 2013 Pearson Education, Inc.

1 A pea flower normally self-pollinates. 3 Pollen from another flower is dabbed on to stigma. Paint brush Stigma Anthers (contains pollen) Ovule 2 Pollen containing structures can be removed to prevent self-fertilization. Tweezers The resulting seeds will contain information on flower color, seed shape and color, and plant height from both parents. Copyright © 2010 Pearson Education, Inc.

© 2013 Pearson Education, Inc. Figure 9. 5

Monohybrid Crosses § A monohybrid cross is a cross between parent plants that differ

Mendelian Genetics: When the Role of Genes Is Clear § Mendel developed four hypotheses from the monohybrid cross: § There alternative forms of genes, called alleles. § For each characteristic, an organism inherits two alleles, one from each parent. § Alleles can be dominant or recessive. § Gametes carry only one allele for each inherited characteristic. © 2013 Pearson Education, Inc.

§ Mendel’s law of segregation § The two members of an allele pair segregate (separate) from each other during the production of gametes. § An explanation of Mendel’s results, including a Punnett square © 2013 Pearson Education, Inc.

© 2013 Pearson Education, Inc. Figure 9. 6 b

§ 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 © 2013 Pearson Education, Inc.

§ 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 © 2013 Pearson Education, Inc.

§ 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? © 2013 Pearson Education, Inc.

Animation: Mendel’s Experiments Click “Go to Animation” / Click “Play” © 2013 Pearson Education,

Quantitative Genetics: When Genes and Environment Interact § 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 © 2013 Pearson Education, Inc.

§Mean: sum up all the phenotypic values and divide by the number of individuals; same as the average. (a) Normal distribution of student height in one college class 5 ft, 10 in (1. 78 m ) Number of men Mean © 2013 Pearson Education, Inc. Bell-shaped curve Variability Height (ft, in) Figure 6. 16 a

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 § Heritability: proportion of the variation within a population due to genetic differences among individuals © 2013 Pearson Education, Inc.

Quantitative Genetics - 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 § Monozygotic twins share all their genes and their environment § Dizygotic twins share environment, but only half their genes © 2013 Pearson Education, Inc.

§Usually influenced by both genes and environment §Monozygotic twins, genetically identical, but different environments

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. © 2013 Pearson Education, Inc.

Genes, Environment, and the Individual 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. © 2013 Pearson Education, Inc.

Genes, Environment, and the Individual 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? © 2013 Pearson Education, Inc.

§ Heritability does not tell us about individual differences. § Heritability is based on variances in populations. § High heritability value for a trait does not automatically mean that most of the difference between two individuals is genetic. © 2013 Pearson Education, Inc.

Genes, Environment, and the Individual 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. © 2013 Pearson Education, Inc.

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 © 2013 Pearson Education, Inc.

§Artificial selection: 2. 6 Average = 2. 6 gallons of milk Selective breeding per day of most productive cow with a bull 3. 2 Average = 3. 2 gallons of milk per day © 2013 Pearson Education, Inc. Figure 6. 19

Genes are segments of DNA that code for ____. A. proteins B. centromeres C.

Prokaryotes typically contain _______. A. single, circular chromosomes B. multiple, circular chromosomes C. multiple,

Which of these events does not contribute to unique combinations of alleles. A. Mutations

True or False: Monozygotic twins occur when two separate eggs fuse with different sperm.

A pea plant has one recessive allele for wrinkled seeds and one dominant allele for smooth seeds. What will the pea plant look like? A. Wrinkle B. Smooth C. Half wrinkled, half smooth D. Not enough information to tell © 2013 Pearson Education, Inc.

Two heterozygotes mate. What are the odds that their offspring will be homozygous recessive?

Does nature or nurture play a bigger role in determining who we are? A.

The Punnett square shown here illustrates the outcome of a cross between a man who carries a single copy of the dominant Huntington’s disease allele and an unaffected woman. What are the odds that Huntington’s disease will not be passed to this offspring? A. 100% B. 75% C. 50% D. 25% © 2013 Pearson Education, Inc.

This Punnett square illustrates the likelihood that a woman who carries the cystic fibrosis allele would have a child with cystic fibrosis if the sperm donor were also a carrier. What are the odds that this offspring will have cystic fibrosis? A. 100% B. 75% C. 50% D. 25% © 2013 Pearson Education, Inc.