Chpt. 17 Genetic Crosses. Gregor Mendal is known as the father of genetics!!!. General Definitions Genetics: is the study of inheritance. Somatic Cells: are all cells except sex cells e.g. Cheek, liver, muscle, blood, leaf, stem etc. Gametes (sex cells): are haploid cells that
Examples of dominant traits:
Dimples in chin
- Cystic fibrosis
Eg. A rabbit with the alleles Ll has long ears because the recessive allele for short ears (l) is prevented from working by the dominant allele for long ears (L).
In rabbits, long ears(L) are dominant over short ears (l). Show the genotypes and phenotypes for the offspring of a cross between two rabbits whose genotypes are (LL) and (ll).
Parents Genotypes LL x ll
Genotypes of Gametes L x l
F1 Genotype of Offspring Ll
F1 Phenotype of offspring All long ears
When answering genetic questions a punnett square is used for the more difficult questions.
Punnett Square: is a grid used to show the ratio of the genotypes of the first generation of offspring (progeny) in a genetic cross.
Six sample questions will be completed involving the use of a Punnett Square (see attached leaflet)
In snapdragons (flower):
- the genotype RR produces red flowers
- the genotype rr produces white flowers
- the heterozygous genotype Rr produces pink flowers
Pedigree: is a diagram showing the genetic history of a group of related individuals.
See pg. 168Question 7
The pattern of sex determination in some species is the reverse of that in humans.
For example in birds, moths and butterflies:
- male body cells – XX
- female body cells – XY
As a result of the this it is the female who determines the sex of the offspring.
Pair of gametes each having only one allele
Each cell has only one chromosome and one allele
Monohybrid Cross:involves the study of a single characteristic e.g. eye colour, seed shape etc.
Dihybrid Cross: involves the study of two characteristics at the same time e.g. studying plant size (tall or small) and pod colour (green or yellow) at the same time.
When gametes are formed .....
either of a pair of alleles ......
is equally likely .......
to combine with either of another pair of alleles .......
A can combine with either Bor b
a can combine with either B or b
Four types of gamete can form
AB Ab aB ab
In pea plants, tall plant (T) is dominant over small plant (t). In addition, green pod (G) is dominant over yellow pod (g).
A tall plant with green pods (homozygous for both traits) is crossed with a small plant with yellow pods.
Why is this a dihybrid cross?
Show by diagrams the genotypes and phenotypes of the progeny of this cross.
In guinea pigs, black coat (B) is dominant to white coat (b). Also short hair (S) is dominant to long hair (s).
Show the genotypes and phenotypes of the F1 progeny for a cross involving a black coated, short-haired guinea pig (heterozygous for both traits) and a white-coated, long-haired animal.
State the expected ratio of the offspring
A homozygous purple-flowered, short stemmed plant was crossed with a red flowered, long stemmed plant.
All the F1 offspring were purple-flowered with short stems.
State the dominant and recessive traits.
Explain, using diagrams, why the F1 plants all had the same phenotypes
Give the expected ratios if an F1 plant is selfed.
Case 1 (Dihybrid Cross):
Heterozygous X Homozygous Recessive
Ratio of offspring = 1 : 1 : 1 : 1
Independent Assortment has occurred!!!
Case 2 (Dihybrid Cross):
Heterozygous X Heterozygous
(both characteristics) (both characteristics)
Ratio of offspring = 9 : 3 : 3 : 1
Independent Assortment has occurred!!!
R and S
r and s
T and t
Pair of alleles
Draw simple chromosome diagrams to illustrate the following cells. In each case show the gametes that might be produced.
a) The genes are not linked and the genotype is AaBb.
b) The genes are linked (A to B and a to b) and the genotype is AaBb.
Draw simple chromosome diagrams to show each of the following cells. In each case indicate the gametes that each cell might produce.
a) Genotype RrSs, genes are not linked
b) Genotype RrSs, genes are linked (R to S and r to s)
c) The genes are not linked, the cell is homozygous for R and heterozygous for S
d) The genes are linked, and the cell is homozygous dominant for both genes.
Show the expected genotypes of the progeny for the following crosses, AaBb x aabb:
a) where there is no linkage
b) where the genes are linked, A to B and a to b
Duchenne muscular dystrophy
Parents GenotypeNXXn nXY
Gametes NX nX nX Y
Offspring Genotype NXnX NXY nXnX nXY
female male female male
carrier normal CB CB
Please read over paragraph relating to this topic pg. 179 in your book.