3.4 Inheritance Flashcards
Gregor Mendel experiments
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)
Mendel’s findings
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
Mendel’s Conclusions
- 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
Law of Segregation
When gametes form, alleles are separated so that each gamete carries only one allele for each gene
Law of Independent Assortment
The segregation of alleles for one gene occurs independently to that of any other gene
Principle of Dominance
Recessive alleles will be masked by dominant alleles
Allele
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
Phenotype
is a structure, process, behaviour or feature of an organism that is determined by its genotype
Genotype
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
Why is the predicted ratio not often the actual ratio?
as the outcome of each mating is independent of the previous
Linked genes
when two genes are on the same chromosome
Unlinked genes
two genes are on different chromosomes
Test cross
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
Incomplete Dominance
where one allele is not completely dominant over the other resulting in the heterozygote being a mixture of the two alleles
Co-Dominance
where both alleles are independently and equally expressed in the heterozygote
Lethal Alleles
are gene mutations that result in a gene product which is not only non-functional but affects the organism survival
Multiple Alleles
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)
Sex-Linked Genes
are genes whose locus is on either the X or Y chromosome
X and Y chromosomes
- 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
Sex-Linked Inheritance Patterns
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
