1. Studying inheritance
Monohybrid inheritance
The genotype is the genetic constitution of an organism. The
phenotype is the expression of this genetic constitution and its
interaction with the environment. There may be many alleles of a
single gene. Alleles may be dominant, recessive or codominant. In a
diploid organism, the alleles at a specific locus may be either
homozygous or heterozygous. The use of fully labelled genetic
diagrams to interpret, or predict, the results of:
• monohybrid and dihybrid crosses involving dominant, recessive
and codominant alleles
• crosses involving sex-linkage, autosomal linkage, multiple alleles
and epistasis.
Use of the chi-squared ( Χ 2 ) test to compare the goodness of fit of
observed phenotypic ratios with expected ratios.
In diploid organisms, characteristics are determined in pairs – only one allele
from each pair can be present in a gamete
Probability and genetic crosses
Actual results of genetic crosses are rarely the same as probability due to the
fact that chance determines which gametes fuse with which
Dihybrid inheritance
2 characteristics determined by 2 different genes on different chromosomes
, 9:3:3:1
Codominance and multiple alleles
Codominance – both alleles are expressed in the phenotype e.g. white flowers
and red flowers might produce pink flowers if they have one of each allele
Multiple alleles – more than 2 alleles e.g. inheritance of ABO blood groups
Sex-linkage
A gene that is carried on the X or Y chromosome
The X chromosome is much longer than the Y chromosome which means that for
most of the length of the X, there is no homologous portion on the Y
chromosome. The characteristics that are controlled by recessive alleles and are
X linked will therefore occur more frequently in males as there is no homologous
portion on the Y for which a dominant allele can counteract the recessive one.
Autosomal linkage
Autosomal linkage - inheritance of two genes which are found on the same
autosome (non-sex-determining chromosome)
Monohybrid inheritance
The genotype is the genetic constitution of an organism. The
phenotype is the expression of this genetic constitution and its
interaction with the environment. There may be many alleles of a
single gene. Alleles may be dominant, recessive or codominant. In a
diploid organism, the alleles at a specific locus may be either
homozygous or heterozygous. The use of fully labelled genetic
diagrams to interpret, or predict, the results of:
• monohybrid and dihybrid crosses involving dominant, recessive
and codominant alleles
• crosses involving sex-linkage, autosomal linkage, multiple alleles
and epistasis.
Use of the chi-squared ( Χ 2 ) test to compare the goodness of fit of
observed phenotypic ratios with expected ratios.
In diploid organisms, characteristics are determined in pairs – only one allele
from each pair can be present in a gamete
Probability and genetic crosses
Actual results of genetic crosses are rarely the same as probability due to the
fact that chance determines which gametes fuse with which
Dihybrid inheritance
2 characteristics determined by 2 different genes on different chromosomes
, 9:3:3:1
Codominance and multiple alleles
Codominance – both alleles are expressed in the phenotype e.g. white flowers
and red flowers might produce pink flowers if they have one of each allele
Multiple alleles – more than 2 alleles e.g. inheritance of ABO blood groups
Sex-linkage
A gene that is carried on the X or Y chromosome
The X chromosome is much longer than the Y chromosome which means that for
most of the length of the X, there is no homologous portion on the Y
chromosome. The characteristics that are controlled by recessive alleles and are
X linked will therefore occur more frequently in males as there is no homologous
portion on the Y for which a dominant allele can counteract the recessive one.
Autosomal linkage
Autosomal linkage - inheritance of two genes which are found on the same
autosome (non-sex-determining chromosome)
