Module 7 - Mendelian Genetics and Extensions

Question 1

Who was Gregor Mendel?

Answer 1

• father of genetics
• he was a monk and scientist working in the 19th century
• he used the pea plant Pisum Sativum to study inheritance patterns

Question 2

What were the advantages of the pea plant?

Answer 2

• considered relatively short generation time (1 year) in his day
• easy to cultivate (used his monastery garden and greenhouse)
• a large number of varieties of peas available, and varieties were genetically pure (homozygous for the trait that he was studying)
• he examined 7 characteristics that appeared in the seeds and the plants grown from the seeds
• pea plants were capable of self-fertilization and could be manipulated to outcross
• to outcross, Mendel opened the buds before anthers (top of a stamen; male reproductive organ, where pollen is located) were fully developed and removed them. He then dusted the stigma (female reproductive organ) with pollen from a different plant

Question 3

What is a gene?

Answer 3

• an inherited factor (encoded in the DNA) that helps determine a characteristic

Question 4

What is an allele?

Answer 4

• one of two or more alternative forms of a gene

Question 5

What is a locus?

Answer 5

• a specific place on a chromosome occupied by an allele

Question 6

What is a genotype?

Answer 6

• a set of alleles possessed by an individual organism

Question 7

What is a homozygote?

Answer 7

• an individual organism possessing two of the same alleles at a locus

Question 8

What is a heterozygote?

Answer 8

• an individual organism possessing two different alleles at a locus

Question 9

What is a characteristic or character?

Answer 9

• an attribute or feature possessed by an organism

Question 10

What is a phenotype or trait?

Answer 10

• the appearance or manifestation of a characteristic

Question 11

What is a monohybrid cross?

Answer 11

• cross between parents that differ only in a single characteristic or trait
• the 1st generation of any cross is called P or P0 (parental) generation. This gives rise to the second generation called F1 (filial 1) generation. The crossing of the F1 plants will give rise to F2 plants and so on

Question 12

What are the conclusions from the monohybrid cross?

Answer 12

1. although F1 plants only show one phenotype from one parent, they must possess genetic factors from both parents because they can transmit both phenotypes to the F2 generation. Thus, F1 plants must possess two genetic factors (alleles) coding for a characteristic
2. alleles must separate when gametes form, and one allele must go to each gamete. When two gametes (one from each parent) are fused together to produce a zygote, the two alleles together form the genotype of the offspring
3. alleles can be dominant or recessive
4. two alleles that encode for a particular characteristic separate with equal probability into the gamete

Question 13

What does homozygous dominant genotype mean?

Answer 13

• two copies of a dominant allele (ex. RR)

Question 14

What does homozygous recessive genotype mean?

Answer 14

• two copies of a recessive allele (ex. rr)

Question 15

What does a heterozygous genotype mean?

Answer 15

• one copy of a dominant allele, and one copy of a recessive allele (ex. Rr)

Question 16

What is the principle of segregation? (Mendel’s first law)

Answer 16

• each diploid organism possesses two alleles with any particular characteristic. The alleles segregate at gametogenesis, and they segregate into gametes in equal proportions

Question 17

What is the concept of dominance?

Answer 17

• when two different alleles are present in a genotype, only the trait encoded by the dominant allele is observed by the phenotype

Question 18

What is the confirmation of principles with the F2 cross?

Answer 18

• one of Mendel’s great strengths was that he used the scientific method:
• he made hypotheses based on initial observations and then conducted additional experiments to test his hypotheses
• those that were heterozygous and homozygous dominant gave round F3 progeny

Question 19

What is probability?

Answer 19

• probability rules allow us to determine the outcome of multifactor (3, 4 and higher) crosses

Question 20

What is the product rule?

Answer 20

• chance of two different events occurring together is the product (multiplication) of their chance of occurring separately

Question 21

What is the addition rule?

Answer 21

• if an event can occur in more than one way, then the chances of that event occurring is the sum (addition) of the individual chances

Question 22

What is a testcross?

Answer 22

• one individual of unknown genotype is crossed with a known homozygous recessive genotype for the traits in question
• when a testcross is performed, any recessive allele in an unknown genotype is expressed because it will be paired with an allele from a homozygous recessive parent. Thus, you will be able to determine the genotype

Question 23

What is a dihybrid cross?

Answer 23

• cross between parents that differ in two characteristics or traits (e.g. shape and colour)

Question 24

Dihybrid cross and probability

Answer 24

• a dihybrid cross between parents that are heterozygous for both traits always have a 9:3:3:1 phenotypic ratio when simple dominance is present
• considering probabilities, such a cross will have:
• 9/16 probability of two dominant traits
• 3/16 probability of the first trait being dominant and the second recessive
• 3/16 probability of the second trait being dominant and the first recessive
• 1/16 probability of both traits being recessive

Question 25

What is independent assortment?

Answer 25

• Mendel found a 9:3:3:1 ratio for a number of dihybrids crosses that he carried out. Using what he found, he derived his second law: principle of independent assortment
• principle of independent assortment: alleles on unlinked (different) loci separate independently of one another

Question 26

What is incomplete dominance?

Answer 26

• incomplete dominance occurs when the phenotype of the heterozygote is intermediate between the two homozygotes

Question 27

What is codominance?

Answer 27

• when two different alleles are equally expressed in a heterozygote. Meaning that the phenotype has products of both alleles

Question 28

What is multiple allele inheritance?

Answer 28

• when there is more than two possible alleles for a particular locus in nature

Question 29

What is sex-linked inheritance?

Answer 29

• autosomes are non-sex chromosomes (1-22 in humans)
• X and Y are sex chromosomes
• individuals either have XX or XY
• X and Y are said to be sex-linked because they segregate with sex chromosomes
• X and Y are not homologues; they are different chromosomes
• different genes are on X vs Y
• for example, an individual has the genotype X^Q Y for the gene Q. Y does not have an allele. Therefore, this is called hemizygous for ‘Q.’
• the ability to see red or green colours is a sex-linked trait