Chapter 8 Inheritance ������������������

CHAPTER

8Inheritance Gregor Mendel was a botanist and monk who now lends his name to the Mendelian laws of inheritance. His study of the patterns of inheritance in pea plants led to our understanding of genetics.

GENETIC TRAITS AND INHERITANCE

Genetic traits can be passed from parent to child. In each somatic human cell, there are 22 pairs of autosome chromosomes. One of each pair is inherited from the mother, the other is inherited from the father.

In the case of complete dominance, an allele for a gene can result in a trait that is dominant or recessive to another trait. An autosomal dominant trait is one where there only needs to be one copy of the allele (on a non-sex chromosome) for the trait to be expressed. An autosomal recessive trait requires two copies of the allele to be expressed.

Inheritance also occurs via the sex chromosomes (X and Y in humans). A female has two X chromosomes, one being inherited from her mother and one from her father. A male has one X and one Y chromosome, the X being inherited from his mother and the Y from his father. In X-linked recessive inheritance, the allele is located on the X chromosome. Because women have two X chromosomes, an allele for a recessive trait is often not expressed. Because men only have one X chromosome, the recessive phenotype is more likely to be expressed. In X-linked dominant inheritance, it is more often females who express the phenotype. Y-linked inheritance is passed from father to son.

PEDIGREES AND PUNNETT SQUARES

A pedigree chart shows the inheritance of a trait over two or more generations. Pedigrees are useful for determining the pattern of inheritance of a particular trait through a family, and can give an insight into the possible genotypes of other family members.

Pedigrees are useful for analysing the genetic history of a family, while Punnett squares are useful for predicting the possible future outcomes of offspring from a genetic cross. Punnett squares consider the alleles of both parents for a particular trait and can be used to calculate the chance of their offspring inheriting the trait. This can be used for both autosomal and sex-linked traits.

MONOHYBRID TEST CROSSES

A monohybrid test cross is similar to a Punnett square, but is used to determine whether an individual is homozygous dominant or heterozygous for a particular trait. This is done by crossing that individual with a known homozygous recessive individual. If any of the offspring show the recessive trait, it can be assumed the unknown parent was heterozygous for that trait.

Dihybrid crosses are also conducted to determine whether two genes are located on the same chromosome (i.e. linked genes) or on different chromosomes (i.e. unlinked genes). This can be worked out from the phenotypic ratios of each pair of traits.