6.2.2 monogenic inheritance

Question 1

define gene

Answer 1

length of DNA (chromosome) that carries instruction for certain protein/polypeptide

Question 2

define allele

Answer 2

alternative form of gene

Question 3

define locus

Answer 3

position on chromosome that carries instructions for particular protein

Question 4

define dominant alllele

Answer 4

expressed when they’re present in genotype (homozygous or heterozygous)

Question 5

define recessive allele

Answer 5

expressed when dominant allele is absent (only when homozygous)

Question 6

define codominant alleles

Answer 6

partially expressed in heterozygous state & result in mixing of character

Question 7

define linkage

Answer 7

genes physically linked together on same chromosome (if closely linked = inherited as group)

Question 8

define sex linked

Answer 8

genes that are carried on sex chromosome (usually X) & whatever is present is expressed in male

Question 9

define cross over

Answer 9

chromatid sections swap between homologous pairs in meiosis, which breaks linkage (more frequent if genes well separated)

Question 10

foundation for genetics

Answer 10

mendel (1886) published results of investigation on genetics:
- studied organism that was easy to grow & cross-fertilise artificially
- worked with 7 characteristics of pea plant
- each characteristic had 2 distinctly contrasting traits (stem height, seed colour, pod shape, pod colour, flower arrangement & flower colour)

Question 11

what was mendel’s simplest experiment

Answer 11

1 characteristic with 1 pair of contrasting traits

Question 12

outline mendel’s simplest experiment

Answer 12

◦ mated individuals from 2 parent strains, each of which showed different phenotype
‣ 1 parent was true-breeding for tall stems
‣ other parent was true-breeding for short
stems
‣ P1 or parental generation
‣ all offspring from this cross (F1
generation) were phenotypic ally
identical to 1 parent type = all tall
◦ allowed members of F1 generation to self-fertilise
‣ produced F2 generation containing = 3/4 tall plants & 1/4 short plants

Question 13

what is the characteristic of height in pea plants

Answer 13

monogenic = governed by 1 gene with 2 distinct alleles T/t)

• t allele (when present in homozygous ‘tt’ individual giving the phenotype) produces phenotypically short plants
• T allele (when present in homozygous ‘TT’ or heterzygous ‘Tt’ individuals) produces phenotypically tall plants
• T allele = dominant
• t allele = recessive

Question 14

what are punnet squares used for

Answer 14

to visualise genotypes/phenotypes resulting from combination of gametes during monhybrid cross (showing potential outcomes of monhybrid inheritance)

Question 15

how to set up punnet squares

Answer 15

• gametes of female parent in vertical column
• gametes of male parent in horizontal column

Question 16

how can we ascertain genotypes of phenotypically similar individuals

Answer 16

using the test cross

Question 17

why may mendel be required to use the test cross to test the genotypes of his pea plants

Answer 17

• short pea plants have genotype tt = shortness is recessive characteristic so must be homozygous recessive
• tall pea plants in F1 generation have same genotype Tt = all heterozygous
• tall plants in F2 generation have genotype TT or Tt = both have same phenotype so impossible to tell genotype from appearance

Question 18

how is the test cross used to test genotypes

Answer 18

• organism displaying dominant phenotype (eg. tall-stemmed pea plant) with unknown genotype (TT or Tt) is crossed with individual showing recessive phenotype (eg. short-stemmed pea plant)
• if any offspring have recessive phenotype, the dominant phenotype is heterozygous (Tt)