7.1 Inheritance Flashcards
What is the genotype?
The genetic constitution of an organism
What is the phenotype?
The expression of the genetic constitution and its interaction with the environment
What are alleles?
• Variations of a particular gene / different DNA base sequence
• Arise by mutation
Dominant alleles
Always expressed in the phenotype
Recessive alleles
Only expressed in the phenotype when two copies are present
Codominant alleles
Both allele expressed in the phenotype
What is meant by homozygous?
Alleles at a specific locus on each homologous chromosome are the same
What is meant by heterozygous?
Alleles at a specific locus on each homologous chromosome are different
What does a monohybrid cross show?
• Inheritance of one phenotypic characteristic coded for by a single gene
• Expected phenotypic ratio of 3:1 (when both parents are heterozygous)
What evidence from a pedigree diagram shows the allele for NAMED PHENOTYPE is dominant?
• Parents with NAMED PHENOTYPE have child without NAMED PHENOTYPE
• So both parents must be heterozygous
What evidence from a pedigree diagram shows the allele for NAMED PHENOTYPE is recessive?
• Parents without NAMED PHENOTYPE have child with NAMED PHENOTYPE
• So both parents must be heterozygous
What does a dihybrid cross show?
• Inheritance of two phenotypic characteristics coded for by two different genes
• Expected phenotypic ratio of 9:3:3:1 (when both parents are heterozygous for both genes)
What is a sex-linked gene?
A gene with a locus on a sex-chromosome
Why are males more likely to express a recessive X-linked allele?
• Females have 2 alleles so only express recessive allele if homozygous recessive
• Males have 1 allele so recessive allele always expressed
What evidence from a pedigree diagram shows the X-linked allele for NAMED PHENOTYPE is recessive?
• Mother without NAMED PHENOTYPE have child with NAMED PHENOTYPE
• So mother must be heterozygous
What evidence from a pedigree diagram shows the gene for NAMED PHENOTYPE is not X-linked?
• Father with NAMED PHENOTYPE has daughter without NAMED PHENOTYPE
• Father would pass allele for NAMED PHENOTYPE onto X chromosome for daughter
OR
• Mother with NAMED PHENOTYPE has son without NAMED PHENOTYPE
• Mother would pass allele for NAMED PHENOTYPE onto X chromosome for son
What is autosomal linkage?
• Two genes are located on the same autosome
• Alleles on the same chromosome stay together during independent segregation so are inherited together
• But crossing over can split these genes, creating new combinations of alleles
What is epistasis?
Interaction between genes where one masks the expression of the other
Why may the observed phenotypic ratio not be the same as the expected?
• Random fertilisation of gametes
• Autosomal linkage / epistasis / sex linkage
• Small sample size so not representative
• Some genotypes may be lethal
