Genetics Using Punnett Squares

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Genetics Using Punnett Squares
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IN 5.10.2010

• Obj We will identify which genetic traits are
  passed from parents to offspring using punnett
  squares.
• Warm-up Which of the following is solely due to
  the genes you have inherited from your parents?
• your height
• your weight
• your language
• your eye color

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What is Genetics?

• Genetics is the scientific study of heredity.
• Heredity is what makes each species unique.
• Passing of traits to offspring from parents or
  ancestors
• Probability is the likelihood that an event will
  occur.
• The principle of probability can be used to
  predict the outcomes of genetic crosses.
• Ex. Coin Flip

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Early Genetics

• The study of genetics
• began with observations made by Gregor Mendel.
• After noticing that the flowers on the pea plants
  were either violet or white, Mendel began to
  study the segregation of heritable traits.

Between 1856 and 1863 he cultivated and tested at
least 28,000 pea plants.
Remember that Mendel worked almost 150 years ago
when nobody knew about genes or even the
structures (chromosomes) that carry genes.
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Here are some traits observed by Mendel
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Lets consider a single gene

• A gene carries information that determines your
  traits. Traits are characteristics you inherit
  from your parents.
• Genes are located in chromosomes.
• Chromosomes come in pairs and there are
  thousands, of genes in one chromosome.

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Continued

• In humans, a cells nucleus contains 46
  individual
• chromosomes or 23 pairs of chromosomes.
• Half of the chromosomes
• come from one parent and half come from the
  other parent.

This is a human karyotype representing the 23
pairs of chromosomes in a male
Here is the detailed structure of a chromosome
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Definitions

• Allele- discrete version of the same gene
• Genotype- the combination of genes of an organism
  for one specific trait
• RR Rr rr
• Phenotype- the physical (outward) appearance of a
  trait in an organism
• Blue eyes tall height dimples

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Definitions

• Dominant trait refers to a genetic feature that
  hides the recessive trait in the phenotype of
  an individual.
• The term "recessive describes a trait that is
  covered over (or dominated) by another form of
  that trait and seems to disappear.
• Homozygous two alleles that are the same for a
  trait (Pure)
• Heterozygous two different alleles for a trait
  (Hybrid)

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Genes, Alleles, Chromosomes
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Mendelian Traits in Humans
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Practice

• We use two letters to represent the genotype. A
  capital letter represents the dominant form of a
  gene (allele) and a lowercase letter is the
  abbreviation for the recessive form of the gene
  (allele).
• Example below Pdominant purple and p recessive
  white

The phenotype for this flower is violet while its
genotype (if homozygous) is PP or Pp (if
heterozygous).
The phenotype for this flower is white while its
genotype is pp (to be white the flower must have
two of the recessive copies of the allele).
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Punnett Square

• Punnett Square A tool developed by Reginald
  Punnett used to predict the number and variety of
  genetic combinations being passed
• from one
• generation
• to another.

Reginald Punnett
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Punnett Squares

• The Punnett square is the standard way of working
  out what the possible offspring of two parents
  will be.
• It is a helpful tool to show allelic combinations
  and predict offspring ratios.

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Before we go further lets review how to set up a
Punnett Square

• We begin by constructing a grid of two
  perpendicular lines.

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• Next, put the genotype of one parent across
• the top and the other along the left side.
• For this example lets consider a genotype of BB
  crossed with bb.

B B

• Notice only one letter goes above each box
• It does not matter which parents genotype goes
  on either side.

b b
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• Next, fill in the boxes by copying the column and
  row head-letters down and across into
• the empty spaces.

B B


b
b
b
B
B
B
b
b
b
B
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Punnett Squares

• Now that we have learned the basics of genetics
  lets walk through some examples using Punnett
  Squares.

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W w
Write the capital letter first.


W W
W w
W
Lets say W- dominant white w- recessive violet
W w
w w
w
Parents in this cross are heterozygous (Ww).
Note Make sure I can tell your capital letters
from lowercase letters. What percentage of the
offspring will have violet flowers? ANSWER 25
homozygous recessive
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• Red hair (R) is dominant over blond hair (r).
  Make a cross between a heterozygous red head and
  a blond.

R r
r r


rr
Rr
rr
Rr
What percentage of the offspring will have red
hair?
50
Homozygous or heterozygous ?
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• Lets try some more
• In pea plants, tall pea plants (T) are
  dominant over short pea plants (t). Construct a
  Punnett Square for a heterozygous tall pea plant
  and a short pea plant.

T t
What are the percentage of phenotypes?


Tt
tt
t t
50 heterozygous tall 50 homozygous short
Tt
tt
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Both parents are able to roll their tongue they
are both heterozygous for tongue rolling. Make a
cross between the parents.
T t
T


TT
Tt
What are the possible genotypes?
t
Tt
tt
25 TT Homozygous 50 Tt Heterozygous 25 tt
Homozygous
What are the possible phenotypes?
75 can roll tongue 25 cant roll tongue
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• Black eyes (B) is dominant over red eyes (b)
• in rats. Make a cross between a homozygous rat
  with black eyes and a rat with red eyes.

B B


What is the possibility of a red eye off springs?
Bb
Bb
b b
Bb
Bb
0 ?
What percentage of the off springs will have
black eyes?
100
Homozygous or Heterozygous ?
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Having dimples is dominant over no dimples. Cross
parents where the father is pure for dimples and
the mother is hybrid for dimples.
D D


DD
DD
D
What are the possible genotypes of the off
springs?
Dd
Dd
d
50 DD, HO (pure) 50 Dd, HE (hybrid)
What are the possibilities of children with
dimples?
100
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Incomplete Dominance
With incomplete dominance, a cross between
organisms with two different phenotypes produces
offspring with a third phenotype that is a
blending of the parental traits.One allele is
not completely dominant over the other. The
letters used would both be capital since
neither trait dominates over the other.
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Codominance

• Both alleles from the parental traits are
  expressed as heterozygous
• For example, red cows crossed with white will
  generate roan cows. Roan refers to cows that have
  red coats with white blotches.
• phenotypic ratio of 1 red2 roan1 white

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Examples of Codominance
This roan horse is heterozygous for coat color.
Its fur appears grey because it has a mixture of
black and white hairs, not because the individual
hair colors are grey.
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Summary

• Complete Dominance

Incomplete Dominance
Codominance
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• Recall Punnett Square is a tool which helps
  predict the possible genetic traits that will
  pass generations.
• Summary DNA is the chemical that contains the
  code for all cell activities including the
  characteristics of offspring.
• Out If you have a homozygous blue-eyed mom and a
  homozygous brown-eyed dad, what would the
  offsprings eyes look like? Explain your
  reasoning in 3 complete sentences.