12.1 Fundamental Concepts of Genetics

Question 1

Each gene may have alternative forms called _____.

Answer 1

Alleles

Question 2

The manifestation of a given genotype as an observable trait is known as a _____.

Answer 2

Phenotype

Question 3

Each human being possesses two copies of each chromosome, called _____. An exception to this are the male sex chromosomes, X and Y.

Answer 3

Homologues

Question 4

Each gene has a particular _____, or location on a specific chromosome.

Answer 4

Locus

Question 5

If only one copy of an allele is needed to express a given phenotype, the allele is said to be _____ and is usually represented with a capital letter.

Answer 5

Dominant

Question 6

If two copies of an allele are needed in order for it to be expressed, the allele is said to be _____ and is usually represented with a lowercase letter.

Answer 6

Recessive

Question 7

A _____ genotype describes a situation in which only one allele is present for a given gene, as is the case for parts of the X chromosome in males.

Answer 7

Hemizygous

Question 8

A situation in which more than one dominant allele exists for a given gene is referred to as _____.

Answer 8

Codominance

Question 9

A population measure defined as the proportion of individuals in the population carrying a particular allele who actually express the genotype is called _____.

Answer 9

Penetrance

Question 10

A related concept to penetrance is called _____, which is defined as varying phenotypes despite identical genotypes.

Answer 10

Expressivity. A disease may show variable expressivity and a range of presentations between no clinical effect and severe disability.

Question 11

What are the four basic tenets of the modern interpretation of Mendel’s first law (of segregation)?

Answer 11

1. Genes exist in alternate forms (alleles).
2. An organism has two alleles for each gene, one inherited from each parent.
3. The two alleles segregate during meiosis, resulting in gametes that carry only one allele for any inherited trait.
4. If two alleles of an organism are different, only one will be fully expressed (dominant) and the other will be silent (recessive). (Keep in mind that codominance and incomplete dominance are exceptions to this rule).