Mendelian inheritance

Question 1

Define Mendelian inheritance

Answer 1

• The basic patterns (or laws) of inheritance that were discovered and described by Gregor Mendel

Question 2

What are the basic patterns of mendelian inheritance?

Answer 2

• Dominance and Recessiveness of ‘units of heredity’ (genes)
• Law of segregation (separation of alleles during meiosis)
• Law of independent assortment (behaviour of non-homologous chromosomes in meiosis)

Question 3

Why was the common garden pea (Pisum sativum) a good choice of research subject?

Answer 3

• Many varieties (different characteristics and traits that can be passed down)
• Well selected traits (either/or situations make it easier to measure and describe)
• Easy to grow (thus providing a large pool of samples and allows for mathematical analysis)
• The structure of the flower gave Mendel strict control over mating and parentage
• Can remove the anthers before the pollen matures, allowing for artificial pollination with a brush

Question 4

What did Mendel do?

Answer 4

• He analysed seven discrete (‘either/or’) traits
• He used reciprocal monohybrid crosses (a single cross between two organisms, then reversing the roles of males and females to confirm results)
• He cross pollinated by changing one trait, and following for up to eight generations

Question 5

What traits did Mendel measure? (D/R)

Answer 5

1. Form of ripe seed (Round/Wrinkled)
2. Colour of seed albumen (Yellow/Green)
3. Colour of flower (Purple/White)
4. Form of ripe pods (Full/Constricted)
5. Colour of unripe pods (Green/Yellow)
6. Position of flowers (Axial/Terminal)
7. Length of stem (Tall/Dwarf)

Question 6

What did Mendel deduce?

Answer 6

• The law of segregation, from the 3:1 ratio
• For each trait there are two copies of each ‘unit of hereditary’ (genes)
• Each alternative form determines the appearance of a specific characteristic
• One form is maternally inherited, the other is paternally inherited
• The quantitative analysis of F2 pea plants disproved the blending theory of hereditary and revealed dominance/recession and the law of segregation
• The 3:1 ratio is due to the underlying mechanism of the way in which genes are separated during meiosis

Question 7

How can pedigree analysis reveal mendelian patterns of inheritance?

Answer 7

• Through the use of the testcross (crossing the test organism with a homozygous recessive and deducing the genotype of the offspring by the phenotype)

Question 8

List three more complex patterns of inheritance

Answer 8

1. Incomplete dominance
2. Codominance
3. Multiple alleles

Question 9

What is incomplete dominance?

Answer 9

• Characterised by heterozygotes having an intermediate phenotype
• Thus, gives three possible phenotypes
• The appearance of the offspring (heterozygote) is BETWEEN the phenotypes of the two parents
• E.g., a pink and red flower = pink offspring

Question 10

What is codominance?

Answer 10

• Characterised by two alleles which are both expressed in the phenotype
• Makes three possible phenotypes
• 2 are parental (homozygous), 1 is heterozygous
• Heterozygote phenotype here is NOT intermediate; it is distinct

Question 11

Explain an important example of codominance

Answer 11

• Human blood groups
• Based on a type of carbohydrate on red blood cell surfaces
• I gene produces an enzyme that ‘glycosylates’ (adds carbohydrate molecules to) specific red blood cell surface proteins
• I gene has two codominant alleles = IA and IB
• IAIA synthesises type A carb
• IBIB synthesises type B carb
• IAIB synthesise both type A and type B carb

Question 12

What does it mean to have multiple alleles?

Answer 12

• Characterised by a gene that exists in more than two allelic forms
• I.e., most human genes
• ABO human blood groups
• I gene has IA, IB and also i allele
• i encodes a non-functional enzyme = no carb on red blood cell surface
• So, ii = type O blood