1. 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?

2. 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.

3. The human life cycle, cont.:
The embryo grow to become a fetus.
After birth, the individual continues to grow until reaching adulthood.

4. 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

5. The Inheritance of Traits - Producing Diversity in Offspring
Mistakes in copying DNA (mutations) produce different versions of genes (alleles), with different results.

6. 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

7. About 8 million different combinations of chromosomes.
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.

8. Siblings share 50% of alleles with each other, on average
Possible sperm cell 1
Possible sperm cell 2
Sperm and egg cells each have only 1 full set—a random combination of maternal and paternal instruction manual pages.
Parent cells have 2 copies of each chromosome—that is, 2 full sets of instruction manual pages, 1 from each parent.
Page 3 Blood-group gene from dad
Page 9 Eye-color genes from mom
Page 3 Blood-group gene from mom
Page 9 Eye-color genes from dad
Siblings share 50% of alleles with each other, on average
Figure 6.6

9. No two humans are genetically identical, except for monozygotic twins.
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.

10. 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.

11. 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

12. 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.

13. Figure 9.5

14. Monohybrid Crosses
A monohybrid cross is a cross between parent plants that differ in only one characteristic.

15. Mendelian Genetics: When the Role of Genes Is Clear
Mendel developed four hypotheses from the monohybrid cross:
There are 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.

16. 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

17. Figure 9.6b

18. 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

19. 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

20. 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?

22. Animation: Mendel’s Experiments
Click “Go to Animation” / Click “Play”

23. 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

24. Mean: sum up all the phenotypic values and divide by the number of individuals; same as the average.
Height (ft, in)
Mean
Bell-shaped curve
(a) Normal distribution of student height in one college class
5 ft, 10 in (1.78 m )
Variability
Number of men
Figure 6.16a

25. 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

26. 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

27. Usually influenced by both genes and environment
Monozygotic twins, genetically identical, but different environments
Figure 6.17

28. 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.

29. 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.

30. 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?

31. 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.

32. 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.

33. 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

34. Selective breeding of most productive cow with a bull
Artificial selection:
Average = 3.2 gallons of milk per day
Average = 2.6 gallons of milk per day
2.6
3.2
Selective breeding of most productive cow with a bull
Figure 6.19

35. Genes are segments of DNA that code for ________.
proteins
centromeres
carbohydrates
karyotypes
Answer: A
Genes code for proteins.

36. Prokaryotes typically contain _______.
single, circular chromosomes
multiple, circular chromosomes
multiple, linear chromosomes
no chromosomes
Answer: A
Most prokaryotes have a single, circular chromosome.

37. Which of these events does not contribute to unique combinations of alleles.
Mutations
Independent assortment
Random fertilization
Cell cycle checkpoints
Answer: D
Cell cycle checkpoints verify whether the processes at each phase have been accurately completed The other three options all contribute to variety in allele combinations.

38. True or False: Monozygotic twins occur when two separate eggs fuse with different sperm.
Answer: B
The correct answer is false because dizygotic twins occur when two separate eggs fuse with different sperm.

39. A pea plant has one recessive allele for wrinkled seeds and one dominant allele for smooth seeds. What will the pea plant look like?
Wrinkle
Smooth
Half wrinkled, half smooth
Not enough information to tell
Answer: B
The dominant allele for smoothness will create a phenotype for smooth.

40. Two heterozygotes mate
Two heterozygotes mate. What are the odds that their offspring will be homozygous recessive?
100%
75%
50%
25%
Answer: D
A Punnett square will show that there is a 25% chance of the offspring being homozygous recessive and a 25% chance of the offspring being homozygous dominant. There is a 50% chance that the offspring will be a heterozygote.

41. Does nature or nurture play a bigger role in determining who we are?
They both play a large role
Answer: C
Most of the traits that characterize us physically and mentally are quantitative traits that are the product of both genes and environment, nature and nurture.

42. 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?
100%
75%
50%
25%
Answer: C
There is a 50% chance that a child will have Huntington’s disease, Hh, and a 50% chance that the child will be unaffected, hh.

43. 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?
100%
75%
50%
25%
Answer: D
There is a 25% chance that the child will have cystic fibrosis, ff. There is a 25% chance that a child won’t have the disease, FF. There is a 50% chance that the child will be a carrier for the disease, Ff.