How can an offspring have a certain trait if neither of its parents displayed that trait?
How many alleles does an offspring get from each parent?
Law of Segregation
passes on ONE
of two alleles
to its offspring.
LAW OF SEGREGATION: Each pair of alleles is segregated, or separated, during the formation of gametes.
Meiosis Aa A a A a Chromosomes double A A a a
Law of Independent Assortment
Alleles for different characteristics are distributed to different gametes independently.
In rabbits, brown fur (B) is dominant to white fur (b). If a rabbit is heterozygous for fur color, what gametes could be created?
A rabbit is homozygous recessive for fur. What color is it?
What percentage of its gametes would have the allele for brown fur?
Chart that shows possible combinations when egg and sperm combine
Draw a square as shown.
Each box represents a possible combination of alleles in the offspring.
Determine which alleles will be in the sex cells of each parent.
Write the egg and sperm possibilities along the top and side.
Copy the alleles into the boxes below and across from them.
Calculate percentage of offspring with each phenotype.
Your problems must display the following:
Resulting phenotypes (# or percentage)
In pot-bellied pigs, grey fur (G) is dominant to pink fur (g).
A homozygous dominant male pig mates with a pink female pig. Predict the possible F 1 offspring.
A man heterozygous for a cleft chin, a dominant trait, mates with a woman with no cleft chin.
What percentage of their children will have a cleft chin?
Pollen from a rose bush with red flowers, a dominant trait, was crossed to a second bush with red flowers. (White is the recessive trait.) If the first plant was homozygous and the second plant was heterozygous, predict the possibilities of their offspring.
In parrots, green feathers are dominant over red feathers. Two heterozygous birds are crossed. What are the possible genotypes of the F 1 generation?
F 1 Crosses
A common cross involves crossing two individuals with opposing traits, one homozygous dominant and one homozygous recessive, the crossing their F 1 offspring to analyze the F 2 generation.
Cross a homozygous dominant and homozygous recessive parrot, then cross two of the F 1 offspring.
What is the ratio of dominant and recessive offspring in the F 2 ?
G = green G = red
A Common Ratio
Any time a homozygous dominant individual is crossed with a homozygous recessive individual, the F 2 generation will have a 3:1 ratio of dominant traits to recessive traits.
Pea Plant Experiments
Experiment 1: Plant Height
Mendel crossed a short
plant with a tall plant.
All offspring were tall.
Crossing two of the offspring resulted in 787 tall plants and 277 short plants – HOW???
Pea Plant Experiments
Experiment 2: Seed Color
Mendel crossed a yellow-seed plant with a green-seed plant.
All offspring had yellow seeds.
Crossing two of the offspring resulted in 6,022 yellow-seed plants and 2001 green-seed plants – HOW???
The crosses we have been completing are called “monohybrid crosses” because they deal with only one trait.
Other Types of Inheritance
Types of Inheritance
Characteristics are determined by more than two alleles
Example: human blood types
(A, B, AB, O)
Heterozygous individuals have a phenotype in-between the dominant and recessive phenotypes
Carnations can be red, white, or pink
R R = red
R’ R’ = white
R R’ = pink
Cross a red carnation with a white carnation. What is the phenotype ratio?
Cross two F 1 carnations from the previous cross. What is the phenotype ratio of the F 2 generation?
Chinchillas are fuzzy, South American rodents. Two alleles control their fur color: F, which represents black, and f, which represents white. Heterozygous chinchillas are grey. Cross a grey chinchilla male with a white female. Give the possible offspring colors and ratio.
Heterozygous individuals display both phenotypes
Human blood type AB
Roan horses = red and white fur
Calico cats = orange and black fur
A roan mare, heterozygous for coat color, is crossed with a red stallion. Describe their offspring. (R codes for red while R’ codes for white.)
A calico cat has the genotype BB’. What is the ratio of F 2 offspring if a pure-breeding black cat (BB) is crossed with a pure-breeding orange cat (B’B’), and then their offspring are crossed?
In an incompletely dominant or co-dominant relationship, crossing two heterozygotes will yield a 1:2:1 phenotype ratio
(Compare this to 3:1 for other crosses)
“ Autosomes” are chromosomes that do not control sex
Humans have 22 pairs of autosomes and 1 pair of sex chromosomes
Many different sex-determination systems
In mammals, X and Y
sex chromosomes exist.
Presence of a Y
Human females are genotype XX.
Males are genotype XY.
Alleles found on the sex chromosomes are said to be sex-linked.