General Biology I - BSC 1010C - Chapter 14: Mendel and the Gene Idea

What is a carrier?

A carrier is an individual who is heterozygous at a given genetic locus for a recessively inherited disorder. The heterozygote is generally phenotypically normal for the disorder but can pass on the recessive allele to offspring.

Give an example of a character and two traits of the character.

A character is a heritable feature that varies among individuals. For example, flower color can vary in pea plants, being either purple or white.

Describe the following terms: dominant allele, recessive allele, homozygous, heterozygous, phenotype, genotype.

A dominant allele is an allele that is fully expressed in the phenotype of a heterozygote. A recessive allele is an allele whose phenotype is not observed in a heterozygote. A homozygous organism has two identical alleles for a gene (i.e. PP or pp), while a heterozygous organism has two different alleles for a gene (i.e. Pp). A phenotype is the observable physical or physiological traits of an organism as determined by its genetic makeup. A genotype is the genetic makeup, or set of alleles, of an organism.

What is a locus and what is an allele? On a homologous pair of chromosomes (that are not duplicated), how many loci and alleles would there be for one gene?

A locus is the location of a gene on a chromosome. An allele is variant of a character or gene. On a homologous pair of chromosomes, there would be 2 loci and 1 allele for one gene.

What is a multifactorial character and what is the norm of reaction for a multifactorial character? Describe the effect the environment can have on a multifactorial genetic disorder.

A multifactorial character means that many factors, both genetic and environmental, collectively influence its phenotype. The norm of reaction for a multifactorial character is the phenotypic range for that character. A multifactorial genetic disorder would be affected by both genetic factors and the environment. For instance, heart disease does not have a single genetic cause and is most likely associated with the effects of multiple genes in combination with lifestyle and environmental factors.

What is a true breed, what is a hybrid? In Mendel's experiment describe the P, first filial, and second filial generation?

A true-breed is an organism that produces offspring of the same variety over many generations of self-pollination. A hybrid is an offspring that results from the mating of individuals from two different species or from two two-breeding varieties of the same species. The P generation is the true-breeding parent individuals, the F1 generation is the first filial hybrid offspring produced by the P generation, and the F2 generation is the second filial offspring resulting from the hybrid F1 generation.

In which phase of meiosis do alleles for a heritable character segregate during gamete formation and end up in different gametes?

Alleles for a heritable character segregate during Metaphase and Anaphase I of meiosis.

Compare and contrast inheritance patterns of recessive and dominantly inherited disorders.

If a disorder is dominantly inherited, the inheritance pattern would indicate that both parents would have the dominant allele (either homozygous dominant or heterozygous). If a disorder is recessively inherited, both parents would likely be heterozygous and may lack symptoms of that disorder.

The results of Mendel's experiment established that alleles for a heritable character segregate during gamete formation and end up in different gametes. Describe the independent variable, dependant variable, control group, and treatment group. Describe how the genotypes (results) of the first and second filial generation supported Mendel's hypothesis.

In Mendel's experiment, the independent factor was producing hybridized offspring vs. producing true-breed offspring. The dependent variable was the observable traits of the offspring. The control group were the pea plants with continued true breeding, and the treatment group were the pea plants with hybridized breeding (crossing two two-breed varieties). Mendel hypothesized that the pea plants in the P generation were homozygous. He did a test-cross with a homozygous recessive plant and a plant with an unknown genotype. This resulted in the F1 being 50% purple (heterozygous) and 50% white (homozygous recessive). This supported Mendel's law of segregation because the flower color alleles segregated randomly into different gametes.

How does incomplete dominance and codominance differ from complete dominance?

In complete dominance, the phenotypes of the heterozygote and the dominant homozygote and indistinguishable (ex. pea plant color). In incomplete dominance, neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of the two parental varieties (ex: snapdragon color). In codominance, the two alleles each affect the phenotype in separate, distinguishable ways (ex: clover patterns).

What does the term epistasis refer to?

In epistasis, the phenotypic expression of a gene at one locus alters that of a gene at a second locus.

At what point in meiosis does independent assortment of homologous chromosomes occur?

Independent assortment of homologous chromosomes occurs in Metaphase I of meiosis.

Why do humans use pedigrees and what are they useful in determining?

It is considered unethical to manipulate the mating patterns of people, so geneticists instead analyze the results of matings that have already occurred by using family pedigrees, diagrams that describe family traits of parents and children across generations. Pedigrees are useful in determining the probability of an inherited disorder in offspring.

Describe Mendel's law of independent assortment. How did he use a dihybrid cross to provide evidence that supported his hypothesis?

Mendel's law of independent assortment states that each pair of alleles segregates independently of each other pair during gamete formation; it only applies to genes on non-homologous chromosomes (non-linked). He worked out this second law of inheritance by following two characters at the same time: seed color and seed shape. Seeds could either be yellow or green, and round or wrinkled. He performed a dihybrid cross with heterozygous pea plants, which resulted in four phenotypic categories with a 9:3:3:1 ratio -- 9 yellow round, 3 green round, 3 yellow wrinkled, and 1 green wrinkled, as predicted. These results supported his hypothesis because the alleles for each tested gene segregated independently from the alleles of the other genes.

Are traits always determined by single genes? If a character is determined by many genes (polygenic) what kind of phenotypes are observed in the population?

No, traits are not always determined by single genes. Two or more genes can be involved in determining a particular phenotype. These two situations are epistasis and polygenic inheritance. In epistasis, the phenotypic expression of a gene at one locus alters that of a gene at a second locus. In polygenic inheritance, two or more genes affect a single phenotypic character. If a character is determined by many genes, there are several different phenotypes observed in the population; for example, three separately inherited skin color genes can have many combinations, resulting in a variety of skin colors.

What is pleiotropy?

Pleiotropy is the ability of a single gene to have multiple effects.

How did Mendel's hypotheses, regarding the flow of genetic information, differ from the blending hypothesis?

The "blending hypothesis" stated that every time organisms reproduced, their genetic information was evenly mixed in their offspring, like paint; if this were the case in Mendel's pea plant experiment, the color of the flowers would be pale purple instead of purple or white. Mendel hypothesized an alternative to the blending hypothesis, saying that genes were passed on from generation to generation in discrete, heritable units, more like marbles mixed together rather than paint.

How are the A, B, O and AB blood types determined? How many alleles are involved in determining these blood types? Is this an example of incomplete dominance or codominance?

The A, B, O, and AB blood types are determined by that person's two alleles of the blood group gene; the three possible alleles are I^A, I^B, and i. The letters refer to two carbohydrates, A and B, that may be found attached to specific cell-surface molecules on red blood cells. An individual's blood cells may have carbohydrate A (type A blood), carbohydrate B (type B), both (type AB), or neither (type O). This is an example of codominance.