Download 3.4 Monohybrid and Test Cross Notes

Monohybrid Cross
IB ASSESSMENT STATEMENT
• Define genotype, phenotype, dominant allele, recessive
allele, codominant alleles, locus, homozygous,
heterozygous, carrier and test cross.
Phenotype & Genotype
• Phenotype – the physical characteristics
of an organism (example: blue eye color)
Phenotype & Genotype
• Genotype – the alleles possessed by an
organism (example: inheriting two blues
alleles on from each parent)
Dominant & Recessive Allele
Dominant Allele – An allele that dominants
over a recessive allele. Represented in
genetics with a capital letter (example the
capital letter B could represent the
dominant allele for brown eyes).
Dominant & Recessive Allele
Recessive Allele - An allele that does NOT
dominate over other traits. Represented in
genetic by a lower case letter (example
the lower case letter b could represent the
allele for blue eyes)
Types of Genotypes
• Homozygous Dominant: An organism
having two identical dominant alleles.
Represented in genetic by having two upper
case letters. (example: BB represents a
person that has two alleles for brown eyes)
Types of Genotypes
• Homozygous Recessive: An organism
having two identical recessive alleles.
Represented in genetic by having two lower
case letters. (example: bb represents a
person that has two alleles for blue eyes)
Types of Genotypes
Heterozygous: An organism having two
different alleles for a trait, one recessive and
one dominant. Represented in genetic by
having one upper case and one lower case
letters. (example: Bb represents a person
that has a dominant alleles for brown eyes
and a recessive allele for blue eyes).
IB Assessment Statement
• Determine the genotypes and phenotypes of the
offspring of a monohybrid cross using a Punnett grid.
The grid should be labelled to include parental
genotypes, gametes, and both offspring genotype and
phenotype.
Punnet Square/ Grid
Punnet Square / Grid is a grid or table that
shows all the possible outcomes between
a cross between a male in female
organisms
Example
• F0: Cross a heterozygote pea plant with smooth seeds with a plant
with wrinkled seeds.
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Select Symbols the alleles:
Write the cross:
Gametes each parent can produce:
Make a punnet square.
F1:
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What percent are heterozygous?
What percent are smooth?
What percent are wrinkled?
P-square Practice Question #1
Let's say that in seals, the gene for the length of the whiskers has two
alleles. The dominant allele (W) codes long whiskers & the recessive allele
(w) codes for short whiskers. Cross two long-whiskered seals, one that is
homozygous dominant and one that is heterozygous?
– The Symbols (letters you will use) ______________________
– The Cross (the parent’s genotypes)_________________________
– The Punnett square:
P-square Practice Question #2
In purple people eaters, one-horn is dominant and no horns is recessive. Draw
a Punnet Square showing the cross of a purple people eater that is
heterozygous for horns with a purple people eater that does not have
horns.
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The Symbols (letters you will use) ______________________
The Cross (the parent’s genotypes)_________________________
The Punnett square:
Test Crosses
Problem:
• When an organism has a shows a
dominant phenotype (example: Brown
Eyes) it is not always possible to know if
this organism is has a heterozygote
genotype (Bb) or Homozygote dominant
genotype (BB) because the phenotype
(physical appearance) it the same for both
genotypes.
Test Cross
Solution:
• To determine the genotype of an
organism showing a dominant phenotype
a test cross is used.
What is a test cross?:
In a test cross an individual that might be
heterozygous or homozygous dominant is
crossed with an individual that is
homozygous recessive.
How it works?
If you cross a homozygous recessive
organism with a homozygous dominant
organism all the offspring will have the
dominant phenotype.
Example: Black fur color is dominant to white fur color in sheep. A
homozygous dominant black sheep is cross with a homozygous
recessive white sheep.
• The Symbols (letters you will use) ______________________
• The Cross (the parent’s genotypes)_________________________
• The Punnett square:
If you cross a homozygous recessive
organism with a heterozygote 50% of the
offspring will have the dominant
phenotype and 50% will have the
recessive phenotype.
Example: Black fur color is dominant to white fur color in sheep. A
heterozygous black sheep is cross with a homozygous recessive
white sheep.
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The Symbols (letters you will use) ______________________
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The Cross (the parent’s
genotypes)_________________________
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The Punnett square: