Ch 11: Pattern of Inheritance

A Smorgasbord of Ears and Other Traits

A.                 The observable traits, such as attached or unattached earlobes, are the result of genetic expression.

B.                 Gregor Mendel was the first person to systematically pursue the questions of genetic.

11.1   Mendel's Insight Into Inheritance Patterns

A.                 Inheritance has always been intriguing to humans. 

1.                 By the late nineteenth century, natural selection suggested that a population could evolve if members showed variation in heritable traits. Variations that improved survival chances would be more common in each generation--in time, the population would change or evolve.

2.                 The theory of natural selection did not fit with the prevailing view of inheritance--blending. 

a.                 Blending would produce uniform populations--such populations could not evolve.

b.                 Many observations did not fit blending--for example, a white horse and a black horse did not produce only gray offspring.

B.                 Mendel's Experimental Approach 

1.                 Gregor Mendel used experiments in plant breeding and a knowledge of mathematics to form his hypotheses.

2.                 Mendel used the garden pea in his experiments. 

a.                 This plant can fertilize itself; true-breeding varieties were available to Mendel.

b.                 Peas can also be cross-fertilized by human manipulation of the pollen.

3.                 Mendel cross-fertilized true-breeding garden pea plants having clearly contrasting traits (example: white vs. purple flowers).

C.                Some Terms Used in Genetics 

1.                 Genes are units of information about specific traits.

2.                 Each gene has a locus on a chromosome.

3.                 Diploid cells have two genes (a gene pair) for each trait--each on a homologous chromosome.

4.                 Alleles are various molecular forms of a gene for the same trait.

5.                 True-breeding lineage occurs when offspring inherit identical alleles, generation after generation; non-identical alleles produce hybrid offspring.

6.                 When both alleles are the same, the condition is called the homozygous condition; if the alleles differ, then it is the heterozygous condition.

7.                 When heterozygous, one allele is dominant (A), the other is recessive (a).

8.                 Homozygous dominant = AA, homozygous recessive = aa, and heterozygous = Aa.

9.                 Genotype is the sum of the genes, and phenotype is how the genes are expressed (what you observe).

10.            P = parental generation; F₁ = first-generation offspring; F₂ = second-generation offspring.

11.2   Mendel's Theory of Segregation

A.                 Predicting Outcomes of Monohybrid Crosses 

1.                 Mendel suspected that every plant inherits two "units" (genes) of information for a trait, one from each parent.

2.                 Mendel's first experiments were monohybrid crosses. 

a.                 Monohybrid crosses have two parents that are true-breeding for contrasting forms of a trait.

b.                 One form of the trait disappears in the first generation offspring (F₁), only to show up in the second generation.

c.                  We now know that all members of the first generation offspring are heterozygous because one parent could produce only an A gamete and the other could produce only an a gamete.

3.                 Results of the F₂ generation required mathematical analysis. 

a.                 The numerical ratios of crosses suggested that genes do not blend.

b.                 For example, the F₂ offspring showed a 3:1 phenotypic ratio.

c.                  Mendel assumed that each sperm has an equal probability of fertilizing an egg. This can be seen most easily by using the Punnett square.

d.                 Thus, each new plant has three chances in four of having at least one dominant allele.

B.                 Testcrosses 

1.                 To support his concept of segregation, Mendel crossed F₁ plants with homozygous recessive individuals.

2.                 A 1:1 ratio of recessive and dominant phenotypes supported his hypothesis.

C.                Mendel's Theory of Segregation 

1.                 The Mendelian theory of segregation states that 2n organisms inherit two genes per trait located on pairs of homologous chromosomes.

2.                 During meiosis the two genes segregate from each other such that each gamete will receive only one gene per trait.

11.3   Independent Assortment

A.                 Predicting Outcomes of Dihybrid Crosses 

1.                 Mendel also performed experiments involving two traits--a dihybrid cross. 

a.                 Mendel correctly predicted that all F₁ plants would show both of the dominant alleles (example: all purple flowers and all tall).

b.                 Mendel wondered if the genes for flower color and plant height would travel together when two F₁ plants were crossed.

2.                 We now know that genes located on nonhomologous chromosomes segregate independently of each other and give the same phenotypic ratio as Mendel observed--9:3:3:1.

B.                 The Theory in Modern Form 

1.                 The Mendelian theory of independent assortment states that during meiosis each gene of a pair tends to assort into gametes independently of other gene pairs located on nonhomologous chromosomes.

2.                 Mendel reported his ideas on heredity to the Brunn Society in 1865 and published them a year later. 

a.                 Few people understood his principles or took note of them.

b.                 He died in 1884 unaware of the revolutionary impact his ideas would have.

11.4   Dominance Relations

A.                 Incomplete Dominance 

1.                 In incomplete dominance, a dominant allele cannot completely mask the expression of another.

2.                 For example, a true-breeding red-flowered snapdragon crossed with a white-flowered snapdragon will produce white flowers because there is not enough red pigment (produced by the dominant allele) to completely mask the effects of the white allele.

B.                 ABO Blood Types: A Case of Codominance 

1.                 In codominance, both alleles are expressed in heterozygotes (for example, humans with both proteins are designated with blood type AB).

2.                 Whenever more than two forms of alleles exist at a given locus, it is called a multiple allele system. In this instance it results in four blood types: A, B, AB, and O.

  

11.5   Multiple Effects of Single Genes

A.                 Sometimes the expression of alleles at one location can have effects on two or more traits; this is termed pleiotropy.

B.                 An excellent example of this phenomenon is the disorder known as Marfan syndrome. 

1.                 The gene for codes for a variant form of fibrillin 1, a protein in the extracellular matrix of connective tissues.

2.                 The altered fibrillin 1 causes a weakening of connective tissues throughout the body.

3.                 Marfan syndrome is characterized by these effects: lanky skeleton, leaky heart valves and weakened blood vessels, deformed air sacs in lungs, pain, lens displacement in the eyes.

11.6   Interactions Between Gene Pairs

A.                 One gene pair can influence other gene pairs, with their combined activities producing some effect on phenotype; this called epistasis.

B.                 Hair Color in Mammals 

1.                 In Labrador retrievers, one gene pair codes for the quantity of melanin produced while another codes for melanin deposition.

2.                 Still another gene locus determines whether melanin will be produced at all--lack of any produces an albino (recessive).

C.                Comb Shape in Poultry 

1.                 Sometimes interaction between two gene pairs results in a phenotype that neither pair can produce alone.

2.                 Comb shape in chickens is of at least four types depending on the interactions of two gene pairs (R and P).

11.7   How Can We Explain Less Predictable Variations?

A.                 Regarding the Unexpected Phenotype 

1.                 Tracking even a single gene through several generation may produce results that are different than expected.

2.                 Camptodactyly (immobile, bent fingers) can express itself on one hand only, both hands, or neither due the possibility that a gene product is missing in one of the several steps along the metabolic pathway.

B.                 Continuous Variation in Populations 

1.                 A given phenotype can vary, by different degrees, from one individual to the next in a population. 

a.                 This is the result of interactions with other genes, and environmental influences.

b.                 In humans, eye color and height are examples.

2.                 Most traits are not qualitative but show continuous variation and are transmitted by quantitative inheritance.

11.8   Environmental Effects on Phenotype

A.                 Fur on the extremities of certain animals will be darker because the enzyme for melanin production will operate at cooler temperatures but is sensitive to heat on the rest of the body.

The color of the floral clusters on Hydrangea plants will vary depending on the acidity of the soil.