4.3 inheritance Flashcards
What is genetics?
The study of mechanisms by which an organism inherits characteristics from its parents
What is the difference between a gene and an allele?
A gene is a section of DNA found at a specific position on a chromosome (locus) that codes for a particular polypeptide
Alleles are versions of the same gene which are found at the same locus on homologous chromosomes
What is the difference between a heterozygous and homozygous genotype?
In a heterozygous genotype the alleles for a given gene are different (e.g. Hh)
In a homozygous genotype the alleles for a given gene are the same (e.g. HH or hh)
What is Mendel’s first law of inheritance?
The law of segregation
Only one of a pair of alleles is present in the gamete
(Each gamete contains only one chromosome from each homologous pair)
What is the difference between a recessive and a dominant allele?
A dominant allele is always expressed within a genotype
A recessive allele is only expressed if both versions of the allele are recessive; it is masked by a dominant allele
What is meant by the term phenotype?
The observable characteristic of an organism
What are codominant alleles?
Alleles which are expressed equally in a phenotype
e.g. blood group AB has both A and B alleles
What is Mendel’s second law of inheritance?
Either member of an allelic pair may combine randomly with either of another pair (provided the genes are not linked) due to independent assortment during metaphase I
What is the ‘F2’ generation in a genetic cross?
The generation produced as a result of selfing two heterozygous individuals from the F1 generation
What are the expected phenotypic ratios within the F2 generation following a monohybrid cross (Yy x Yy)?
3:1
3 with dominant characteristic, 1 with recessive characteristic
What is the ‘F1’ generation in a genetic cross?
The first generation produced as a result of breeding two homozygous individuals, one with the dominant characteristic(s), the other with the recessive characteristic(s)
All offspring are heterozygous, with the dominant trait(s)
What is a linked gene?
Genes located close together on the same chromosome that tend to be inherited together
What are the expected phenotypic ratios within the F2 generation following a dihybrid cross (YyRr x YyRr)?
9:3:3:1
9 with both dominant characteristics, 3 with either dominant characteristic, 1 with both recessive characteristics
What do Mendel’s laws of inheritance assume with regard to dihybrid inheritance?
The genes are not linked and are located on different chromosomes
How is a test cross carried out?
Cross the organism with the dominant characteristic with an individual with a recessive genotype.
If all of the offspring have the dominant characteristic, the unknown parent is homozygous.
If only 50% of the offspring have the dominant characteristic, the unknown parent is heterozygous
Why are test crosses carried out?
To determine whether a dominant characteristic observed in an organism is determined by one or two dominant alleles (homozygous dominant or heterozygous)
State the expected results if a dihybrid test cross is carried out on a doubly heterozygous individual (YyRr x yyrr)
The offspring show a phenotypic ratio of 1:1:1:1
Explain why the expected phenotypic ratio from a dihybrid test cross between a doubly heterozygous individual (YyRr) and a doubly recessive individual (yyrr) may not be 1:1:1:1
This is due to crossing over of linked genes in prophase I
The recombinants will be present in lower numbers
In dihybrid inheritance of linked genes, state the phenotypic ratio which is displayed from a cross between two doubly heterozygous individuals
It shows a 3:1 ratio not the expected 9:3:3:1 ratio shown with unlinked genes
Draw a diagram showing crossing over of linked genes in prophase I, between two individuals, one doubly homozygous dominant and the other doubly recessive
What test can be carried out to investigate if the difference between expected and observed results from a dihybrid cross is due to chance alone or a significant event?
Chi-squared
Mendelian genetics is based on the assumption that genes are not linked.
Significant deviation from the expected results suggests that genes are linked
How are the ‘degrees of freedom’ calculated?
The number of categories minus one (n-1)
In this case, degrees of freedom = 3 (4-1)
How are chi-squared results interpreted?
If the chi-squared value is greater than the critical value (at 0.05 probability) then the null hypothesis is rejected.
The difference between observed and expected results is significant and not due to chance alone
In a dihybrid test cross this suggests genes are linked