3.4 Inheritance

Question 1

Gregor Mendel experiments

Answer 1

First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics
These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)

Question 2

Mendel’s findings

Answer 2

When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed
When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1

Question 2

Mendel’s Conclusions

Answer 2

• Organisms have discrete factors that determine its features (genes)
• Organisms possess two versions of each factor (alleles)
• Each gamete contains only one allele (haploid)
• Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm
• For each factor, one version is dominant over another and will be completely expressed if present

Question 3

Law of Segregation

Answer 3

When gametes form, alleles are separated so that each gamete carries only one allele for each gene

Question 4

Law of Independent Assortment

Answer 4

The segregation of alleles for one gene occurs independently to that of any other gene

Question 5

Principle of Dominance

Answer 5

Recessive alleles will be masked by dominant alleles

Question 6

Allele

Answer 6

one specific form of a gene, differing by one or a few bases and occupying the same gene locus as other alleles of the gene

Question 7

Phenotype

Answer 7

is a structure, process, behaviour or feature of an organism that is determined by its genotype

Question 8

Genotype

Answer 8

of an organism refers to its genetic makeup
heterozygous: has two different alleles
homozygous: when both chromosomes have identical copies of the allele for a gene

Question 9

Why is the predicted ratio not often the actual ratio?

Answer 9

as the outcome of each mating is independent of the previous

Question 10

Linked genes

Answer 10

when two genes are on the same chromosome

Question 11

Unlinked genes

Answer 11

two genes are on different chromosomes

Question 12

Test cross

Answer 12

by looking at a dominant phenotype you are unable to tell whether the organism is homozygous dominant or heterozygous
so you mate the unknown genotype with an organism showing the recessive trait
if any of the offspring show the recessive trait then the original unknown parent was heterozygous
if no recessive trait shows up then the parent was homozygous

Question 13

Incomplete Dominance

Answer 13

where one allele is not completely dominant over the other resulting in the heterozygote being a mixture of the two alleles

Question 14

Co-Dominance

Answer 14

where both alleles are independently and equally expressed in the heterozygote

Question 15

Lethal Alleles

Answer 15

are gene mutations that result in a gene product which is not only non-functional but affects the organism survival

Question 16

Multiple Alleles

Answer 16

is when there is more than one allele that codes for one trait
e.g. blood types ABO (A&B are co-dominant and both dominant over O)

Question 17

Sex-Linked Genes

Answer 17

are genes whose locus is on either the X or Y chromosome

Question 18

X and Y chromosomes

Answer 18

• The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes)
• The X chromosome is longer and contains many genes not present on the Y chromosomes (153 million bp ; ~ 2,000 genes)
• Hence, sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome

Question 19

Sex-Linked Inheritance Patterns

Answer 19

differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY)
As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease)
Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits
X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele