6.1.2 - Patterns of Inheritance

Question 1

What is monogenetic inheritance?

Answer 1

Inheritance of a single gene

• eg mendel’s experiments on pea pods - either green (GG) or yellow (gg)

Question 2

How do you lay out a genetic cross?

Answer 2

1) state phenotype of parents
2) state genotype of both parents (assign letter)
3) state gametes of each parent (common practice to circle these)
4) use a punnet square to show the results of the random fusion of gametes during fertilisation
5) state the proportion of each genotype (% usually best)
6) state the corresponding phenotype for each possible genotype

Question 3

What is codominance?

Answer 3

Two different alleles occur for a gene, both of which are equally dominant
Both alleles are expressed in phenotype if present

• eg snapdragon - an allele that codes for red flowers + an allele that codes for white flowers = pink flowers

Question 4

What are the conventions of codominant allele atterns of inheritance?

Answer 4

• we cant use upper and lower case to represent alleles bcs there isn’t a recessive/dominant allele
• we use different letters to represent the 2 alleles (eg R/W)
• we write them as superscripts on a letter that represents the gene (eg C for colour)

Question 5

What are multiple alleles?

Answer 5

Genes which have more than 2 versions
However, organisms can only carry two versions of the gene

EG BLOOD TYPE

Alleles either A (I^A), B (I^B) or O (i)
A and B are codominant, O is recessive to both

4 possible blood types:
•blood group A - AA or AO
•blood group B - BB or BO
• blood group AB - AB
•blood group O - OO

Question 6

What is sex linkage?

Answer 6

• characteristics determined by genes carried on sex chromosomes - these genes are ‘sex-linked’
• Y chromosome is much smaller than X chromosome so there are a number of genes in the X chromosome, which is not present on Y chromosome, will occur more frequently in males

Question 7

What are dihybrid crosses?

Answer 7

Studies or inheritance of 2 different characteristics at a time (2 genes on different chromosomes)

Eg Mendel’s hybrid cross, with traits seed shape and seed colour

Question 8

What is the chi-squared test?

Answer 8

Statistical test to find out if the difference between the observed and expected data is small enough to be due to chance and therefore not significant

Null hypothesis: offspring are close enough to the expected ratio

Larger value = greater (significant) difference

Can be used:
• for categorical data
• where there is a strong biological theory used to predict expected values

Needs:
•relatively large sample size
• raw counts (not processed data eg percentages)
•no 0 scores

Question 9

What factors need to be taken into account when doing a chi-squared test?

Answer 9

• differences may be positive or negative
• dividing by E (expected frequencies) takes into account the size of numbers
• sum sign takes into account the number of comparisons being made