Download Intro to punnett squares and problems

The study of heredity
Heredity and environment are two factors
that influence the development of living
things
Heredity is the passing on of traits from
parents to offspring
Genetics is the study of heredity and how
genes work in this process.
Gregor Mendal's conclusions.
1.Genes occur in pairs. (Remember: genes are
just regions on a chromosome that code for a trait.
Chromosomes are made of DNA.)
2. Genes display dominance and recessiveness.
- Since genes occur in pairs, usually only
one gene will show its trait.
Dominant gene = the gene whose trait will be
expressed in an individual. Dominant genes are
noted by capital letters.
example:
If tall is dominant over short in pea plants,
then the plant with either of these two gene
pairs (TT or Tt) will be tall.
Recessive gene =
- Does not show its trait if the other gene in the
pair is dominant.
- This gene is overpowered by the dominant gene.
- Noted by using small letters (t).
- Only if both genes in a gene pair are recessive
will the recessive trait be expressed.
example: Tt = tall plant
TT = tall plant
tt = short plant (because of two recessive
genes that code for short plants)
3. The law of segregation:
When gametes are formed only one gene of
a pair will be found in that gamete. (a
result of meiosis).
4. The law of independent assortment:
When genes pairs separate they go to
gametes independently of other gene
pairs.
example:
The genes for eye color are not related to
genes for hair color, or height, etc.
Combinations of genes could be:
Homozygous - Both genes the same - either
dominant or recessive (TT or tt)
- true-breeding, purebred
Heterozygous - Genes are different (Tt)
- hybrid
Phenotype - What an individual looks like
Genotype - Genes that an organism possesses
Genes occur in pairs... On homologous
chromosomes
T
t
Could be:
•Dominant
•Recessive
- Represented by capital and lower case
letters
DNA replication
T
T
t
t
Then...
Meiosis
Gametes - with only one copy of the gene
Punnett square shows:
- Results of meiosis (gametes) &
-POSSIBLE results of fertilization
Parent 1
T
Parent
2
T
t
t
TT
Tt
Tt
tt
How to Solve a Genetics Problem
• Read the problem and determine the
dominant trait(s)
• Make a key to describe what genes
your letters represent. Ex. T = tall t
= short
• Determine genotype of parents
• Determine possible gametes formed
by parents
• Draw a punnett square
• Place gametes of each parent on
top and along left side
• Show results of fertilization by
filling in boxes
• List results = Genotype &
Phenotype
Monohybrid Cross
In pea plants tall is dominant over short. Cross a
homozygous tall plant with a heterozygous tall
plant. What are the possible outcomes?
Freckles are dominant in humans. Cross
a purebred freckled man with a woman
who does not have freckles.
In chinchillas black is dominant over
grey. Cross two heterozygous
individuals.
Dihybrid Cross
In pea plants tall plants and yellow seeds are dominant over short plants
and green seeds. Cross a heterozygous tall yellow plant with a
homozygous tall green plant. What are the possible outcomes?
Punnett square:
Key:
Parents Genotypes:
_______ X ________
Determining
Gametes
Results:
Cross a homozygous tall, yellow plant with a short,
green plant. What could the F2 generation of this cross
look like?
Intermediate Inheritance
In some cases there are no dominant
genes. Or equally dominant genes. As
a result, a cross may result in:
- a true blending of the characteristics
- both characteristics being present
Many traits are incompletely dominant
over others, which result in intermediate
forms.
In guinea pigs the straight hair shape is dominant
and curly hair is recessive. Wavy hair is also a
phenotype. Cross a guinea pig with straight hair
to a curly haired guinea pig, and list the results.
In snap dragons, red in incompletely dominant
over white. Pink is also a phenotype. Cross a
white flower with a pink flower and list the
results.
In horses, roan color is the result of
codominant genes both showing up.
Individual hairs are either one color or the other.
Sex-linked traits
• traits whose genes are located on the X
chromosome
• alleles are shown by
XN = normal and Xn = affected
• show up more often in males
There is no second X chromosome to
contain a dominant gene.
Red green Colorblindness is a sex-linked trait. Cross a
carrier female with a normal male. What are the
possible results?
Multiple alleles
• Genes with more than 2 alleles
• Most commonly used example is blood types
IA, IB, i
A person with blood type AB marries a person with blood type
O. What are the possible blood types of their children?
2. Cross a person with an AB blood type
with an AO blood type and list the results.
3. A woman with type B blood is accusing
a man with type A blood of being the father
of a child with type O blood. Will the
courts be able to prove that the man is not
the father of the child? Is it possible that
the man is the father of the child? Do a
cross and list the results to justify your
answer.