Topic Eight - Patterns of inheritance Flashcards
What is a pedigree chart?
A pedigree chart is a visual representation of a family’s genetic history that depicts the inheritance of a particular trait.
What does a pedigree chart use?
It uses symbols to represent individuals and their relationships across generations.
Drawing a human pedigree involves using specific symbols, such as squares for males and circles for females.
Each individual in a pedigree can be identified using a generation number (I, II) and a position within the generation (1, 2).
Using Pedigrees to Determine Genotypes
To draw conclusions about inheritance, pedigrees should ideally include an adequate number of affected and unaffected individuals, both sexes, and multiple generations.
The pattern of inheritance in a pedigree may reveal information about the trait, including whether it’s controlled by an autosome or a sex chromosome.
Autosomal Dominant Inheritance
Autosomal dominant traits appear in both males and females in roughly equal numbers across multiple generations.
All affected individuals have at least one affected parent.
Once the trait disappears from a branch of the family tree, it does not reappear.
Fathers and mothers can both pass the condition to their sons and daughters.
Autosomal Recessive Inheritance
Autosomal recessive traits affect both males and females equally across generations.
Two unaffected parents can have an affected child.
Offspring of parents who both have the trait will all be affected.
The trait can “skip” generations, disappearing and reappearing in later generations.
X-Linked Dominant Inheritance
A male with an X-linked dominant trait passes it on to all his daughters and none of his sons.
Every affected individual has at least one parent with the trait.
Once the trait disappears from a branch of the family tree, it does not reappear.
More affected females than males are typically seen in a large pedigree.
X-Linked Recessive Inheritance
Affected females with X-linked recessive traits always have affected sons.
Daughters of affected males are carriers of the trait.
All children with two affected parents will exhibit the trait.
More males than females are affected across a large pedigree.
Y-Linked Traits Inheritance
Y-linked traits are only expressed in males.
An affected male passes the trait to his sons.
Y-linked traits do not skip generations.
How do you represent an unknown trait?
Individuals with unknown genotypes for a dominant trait may be represented as B–
Monohybrid Cross and Test Cross
Monohybrid crosses involve one gene.
Test crosses determine the genotype of an individual with a dominant phenotype.
In a cross between a heterozygote and a homozygous recessive, the ratio of phenotypes helps deduce the unknown genotype.
Codominant Alleles
Codominance occurs when both alleles of a heterozygous organism are expressed in its phenotype.
Example: Roan cattle (CR/CW) have a mixture of red and white hairs.
Punnett squares can illustrate outcomes of codominant crosses.
Lethal Genes
Some genotypes cause early embryonic death.
Lethal genotypes don’t appear in offspring.
Example: Ay (yellow coat) allele in mice is lethal in homozygous condition.
Lethal genes can affect expected phenotypic ratios.
Monohybrid Test Crosses
A monohybrid test cross involves an individual of uncertain genotype and a homozygous recessive individual.
It helps determine if a dominant phenotype is heterozygous or homozygous.
Test crosses reveal genotype by comparing offspring phenotypes.
Predicting Genetic Outcomes for Linked Genes
Linked genes are on the same chromosome.
Test crosses with known linkage can predict offspring outcomes.
Proportions of parental and recombinant offspring indicate linkage.
Estimating Distance Between Linked Genes
Recombinant offspring percentages help estimate distance between linked genes.
Percentage of recombinant offspring corresponds to map unit distance between genes.
Closer genes are more tightly linked and show fewer recombinants.
