7.1 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?

 

    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.

 

    The human life cycle, cont.:

   The embryo grow to become a fetus.

   After birth, the individual continues to grow until reaching adulthood.

 

    Genes are segments of DNA that code for proteins.

   Analogous to words in an instruction manual for building a human

    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

 

7.1 The Inheritance of Traits - Producing Diversity in Offspring

    Mistakes in copying DNA (mutations) produce different versions of genes (alleles), with different results.

    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

    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.

    Due to independent assortment, the instructions in one sperm cell is an unique combination of pages.

    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.

 

7.2 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.

    Mendels principles also apply to many genetic diseases in humans.

 

    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

 

7.2 Mendelian Genetics - Genetic Diseases in Humans

    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

 

    Huntingtons 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

 

7.2 Mendelian Genetics - Using Punnett Squares to Predict Offspring Genotypes

    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?

 

    Dihybrid crosses are crosses that involve two traits.

    The first step in a dihybrid is to determine the possible gametes.

    Yellow (Y) is dominant to green (y) and Round (R) is dominant to wrinkled (r).

    If you cross YyRr x YyRr,

   Possible gametes for parent 1 are YR, Yr, yR, yr

   Possible gametes for parent 2 are YR, Yr, yR, yr

 

7.3 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

 

    Mean: sum up all the phenotypic values and divide by the number of individuals; same as the average.

    Variance: a measure of how much variability there is in the population

 

7.3 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

 

7.3 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

   Heritability of IQ from such twin studies estimated to be about 0.52

 

    Twins share a more similar environment than most humans

    Similar treatment of twins might explain why their IQs are so similar

    Monozygotic twins raised apart share all genes but are treated like everyone else

    Estimates of IQ heritability for such twins is 0.72

    Drawback: limited number of such twins to study

    Table 7.2 discusses IQ heritability

 

7.4 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.

 

    Allow the mice in each group to breed, maintaining their diets.

    Measure the weight of adult offspring; correlation with parents shows high 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?

 

    A highly heritable trait can still respond to environmental change.

    Maze-learning ability is highly heritable in rats.

   Bright rats have bright offspring

   Dull rats have dull offspring

    Still, no rats learned well in a restricted environment.

   All rats learned better in an enriched environment.

 

    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.

 

7.4 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.