Term 2 Lecture 12: Basic principles of heredity

Question 1

Gregor Mendel monitored 7 characteristics of his pea plants

Answer 1

• pod colour yellow/green
• pod shape inflated/ constricted
• seed colour yellow/green
• seed shape smooth/ wrinkled
• seed coat white/ gray
• stem length short/tall
• flower position: axial -along the stem terminal -at tip of stem

Question 2

Key terms

Answer 2

gene- an inherited factor coded in DNA that helps to determine a characteristic

allele - one of two or more alternative forms of a gene

locus - specific position on a chromosome occupied by an allele

genotype - set of alleles possessed by an individual organism

heterozygote - an individual with two different alleles at a locus

homozygote - an individual with two of the same alleles at a locus

phenotype or trait - appearance/ manifestation of a characteristic

characteristic/ character - attribute/ feature of an organism

Question 3

Aa

Answer 3

Dominant allele capital letter
Recessive allele lower case letter

(both in italics)

Question 4

A typical Mendel experiment: monohybrid cross

Answer 4

following one trait hence mono

He did this by first deliberately pollinating two homozygous plants together and preventing self fertilisation. He then grew these plants and harvested the seeds. This generation is therefore known as the P or parent generation.

The P plants produce homozygous seeds
One plant makes RR - round and the other rr- wrinkly

These seeds are grown and the plants crossed to produce F1 offspring. All F1 offspring are Rr - round.

When F1 offspring plants are self fertilized 3/4 have round seeds and 1/4 have wrinkled seeds
(Genotypes: RR,Rr,Rr, rr)

Conclusion: the traits of the parent plants do not blend. Although F1 plants display the phenotype of one parent both traits are passed to F2 offspring in a 3:1 ratio.

Question 5

Mendels conclusions from monohybrid crosses

Answer 5

Monohybrid cross: a cross between 2 parents that differ in a single characteristic e.g. tall/short plant or round/wrinkled seeds

conclusions:
1) one character is encoded by two genetic factors (alleles)
2) the 2 alleles separate when gametes are formed
3) the concept of dominant and recessive traits
4) 2 alleles separate with equal probability into the gametes

Question 6

Mendels first law: Principle of segregation

Answer 6

Each individual diploid organism possesses 2 alleles for any one trait. These 2 alleles segregate when gametes are formed and one goes into each gamete

Question 7

The concept of dominance

Answer 7

When 2 different alleles are present in a genotype, only the trait encoded by one of them, the ‘dominant’ allele is observed in the phenotype (denoted by capital italic letter)

Question 8

Sutton: chromosomal theory of heredity supported Mendels findings

Answer 8

By back crossing an F1 progeny with a parental genotype Mendel established dominance was hereditary

Question 9

Dihybrid crosses

Answer 9

Examine 2 traits at the same time
expected F2 outcome: 9:3:3:1

Question 10

Comparison of principle of segregation and independent assortment

Answer 10

Segregation
1) each organism possesses 2 alleles per trait (before meiosis)
2) alleles separate when gametes are formed - anaphase l
3) alleles separate in equal proportions - anaphase l

Independent assortment
Alleles at different loci separate independently- Anaphase l

(^this assumes no crossing over occurs. If crossing over does occur then segregation/ assortment may also occur in anaphase of meiosis ll)