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Mendelian Genetics
Creating Gametes
Probability
Genetic Terms
Gregor Mendel
Summarize Gregor Mendel’s contributions to the study of genetics
• Performed cross-pollination
experiments on pea plants to
calculate ratios in which traits
appeared
• Discovered basic principles of
heredity
• Considered the father of
genetics
Creating Gametes
Differentiate between the laws of segregation and independent assortment
• Law of segregation – the law stating
that members of each pair of genes
separate, or segregate, when gametes
are formed
– Each gamete formed during meiosis
receives one member of each
chromosome pair
Creating Gametes
Differentiate between the laws of segregation and independent assortment
• Law of independent assortment – the law that describes
genes encoding for different traits separating from one
another independently during the formation of gametes
Probability
Calculate probabilities and relate how probabilities influence the production of gametes
• Probability – the fraction or percentage that describes the
likelihood of an event taking place
• Probability of a coin flip
– 50% chance of landing on heads, 50% chance of landing on tails
– Probability is a prediction, not a guarantee
– Each flip is an independent event with the same probability
Probability
Calculate probabilities and relate how probabilities influence the production of gametes
• Product rule – the rule that states that the probability of
two or more independent events occurring together is the
product of the individual probabilities of each event
occurring alone
• Using the product rule
Step 1
Step 2
Determine the probability of each event.
Multiply the probabilities together to find the
probability of both events occurring.
Probability Example
Calculate probabilities and relate how probabilities influence the production of gametes
Ex) What is the probability of simultaneously flipping a coin that
lands on heads and spinning the color red on a four-colored
spinner?
Step 1
Determine the probability of each event.
Step 2 Multiply the probabilities together to find the probability of both
events occurring.
There is a one out of eight (12.5%) chance of flipping heads and
spinning red simultaneously.
Probability
Calculate probabilities and relate how probabilities influence the production of gametes
• When applying probability to genetic crosses, geneticists
assume
– A pair of genes will segregate in a 1:1 ratio during meiosis
• (Law of segregation)
– Gametes combine randomly during fertilization
Genetic Terms - Define major genetic terms
• Character – a feature
present in an organism’s
appearance
– Ex) Fur color in mice
• Trait – a different
version of a character
– Ex) White or black fur
• Gene – the unit of
hereditary information
consisting of a specific
nucleotide sequence
within DNA
• Allele – the alternate
form of a gene
Genetic Terms
• Dominant – an allele that is fully expressed
in the phenotype
– Represented by a capital letter
• Recessive – an allele that is masked in the
phenotype
– Represented by a lowercase letter
Genetic Terms
• Genotype – the genetic
makeup of an organism
– Written as two letters
• Phenotype – the physical
and physiological traits
of an organism
– Descriptive
Genetic Terms
• Homozygous – the state of having two of the same alleles
for a given genetic character
– Ex) Homozygous dominant genotype RR results in phenotype
red flowers
• Heterozygous – the state of having two different alleles for
a given genetic character
– Ex) Heterozygous genotype Rr results in phenotype red flowers
Mendelian Genetics
Pedigrees
Pedigrees
Interpret and analyze genetic pedigrees
• Learning about human genetics
requires researching previous matings
• Pedigree – a family tree describing the
occurrence of heritable characters in
parents and offspring across
generations
Pedigrees
• Widow’s peak results from a
dominant allele
• Pedigrees can help predict future
offspring
Pedigrees
• Analyzing a pedigree
Step 1 Determine if the trait in the pedigree is dominant or
recessive.
Step 2 Determine the genotypes of the individuals.
Step 3 Use the genotypes to create a Punnett square to solve the
problem.
Pedigrees Example Part 1
Ex) Some people have attached earlobes while others have free
earlobes. Analyze the provided pedigree that traces this trait in a
family. First, determine if the allele for attached earlobes is
dominant or recessive. Then, calculate the probability of the
couple in the second generation having a third child with
attached earlobes.
Step 1
Determine if the trait in the pedigree is dominant or recessive.
Attached earlobes is recessive.
Pedigrees Example Part 2
Ex) Some people have attached earlobes while others have free
earlobes. Analyze the provided pedigree that traces this trait in a
family. First, determine if the allele for attached earlobes is
dominant or recessive. Then, calculate the probability of the
couple in the second generation having a third child with
attached earlobes.
Step 2
Determine the genotypes of the individuals.
Ee x Ee
Pedigrees Example Part 3
Ex) Some people have attached earlobes while others have free
earlobes. Analyze the provided pedigree that traces this trait in a
family. First, determine if the allele for attached earlobes is
dominant or recessive. Then, calculate the probability of the
couple in the second generation having a third child with
attached earlobes.
Step 3 Use the genotypes to create a Punnett square
to solve the problem.
25% chance of having a child with
attached earlobes
Do these results make sense? Yes.
Mendelian Genetics
Punnett Squares
Punnett Squares
Solve Punnett square problems focusing on Mendelian genetics
• Punnett square – a chart used to visualize the possibilities
of a genetic cross
– Organizes possible gamete configurations in order to calculate
genotype and phenotype ratios
• Probability ≠ guarantee
Punnett Squares
• Solving genetics problems using Punnett squares
Step 1
Draw and label Punnett square with parent genotypes.
Step 2
Use parent genotypes to find each box’s genotype.
Step 3
Determine genotypic percentages and ratios.
Step 4
Use dominance to determine phenotype in each box.
Step 5
Determine phenotypic percentages and ratios.
Punnett Squares
• A single-trait cross focuses on one character
• Monohybrid cross – a genetic cross involving individuals
both heterozygous for one trait
Punnett Squares Example Part 1
Ex) In mice, the allele for black fur color (B) is dominant over
the allele for white fur color (b). A homozygous dominant black
mouse mates with a white mouse. Determine the expected
genotype and phenotype percentages of the offspring.
Step 1
Draw and label a Punnett square with parent genotypes.
Punnett Squares Example Part 2
Ex) In mice, the allele for black fur color (B) is dominant over
the allele for white fur color (b). A homozygous dominant black
mouse mates with a white mouse. Determine the expected
genotype and phenotype percentages of the offspring.
Step 2
Use parent genotypes to find each box’s genotype.
Step 3
Determine genotypic percentages and ratios.
100% Bb
Punnett Squares Example Part 3
Ex) In mice, the allele for black fur color (B) is dominant over
the allele for white fur color (b). A homozygous dominant black
mouse mates with a white mouse. Determine the expected
genotype and phenotype percentages of the offspring.
Step 4
Use dominance to determine phenotype in each box.
Step 5
Determine phenotypic percentages and ratios.
100% black fur color
Does the result make sense? Yes.
Punnett Squares Example Part 1
Ex) In a particular species of flower, red flower color (R) is
dominant over white flower color (r). A heterozygous red flower
is crossed with another heterozygous red flower. What are the
chances of producing offspring with white flowers from this
particular cross?
Step 1
Draw and label Punnett square with parent genotypes.
Punnett Squares Example Part 2
Ex) In a particular species of flower, red flower color (R) is
dominant over white flower color (r). A heterozygous red flower
is crossed with another heterozygous red flower. What are the
chances of producing offspring with white flowers from this
particular cross?
Step 2
Use parent genotypes to find each box’s genotype.
Step 3
Determine genotypic percentages and ratios.
25% RR
50% Rr
25% rr
1 RR :
or
2 Rr :
1 rr
Punnett Squares Example Part 3
Ex) In a particular species of flower, red flower color (R) is
dominant over white flower color (r). A heterozygous red flower
is crossed with another heterozygous red flower. What are the
chances of producing offspring with white flowers from this
particular cross?
Step 4
Use dominance to determine phenotype in each box.
Step 5
Determine phenotypic percentages and ratios.
75% red or 3 red :
1 white
25% white
25% chance of offspring with white flowers
Does the result make sense? Yes.
Punnett Squares
• A double-trait cross focuses on two characters
• Dihybrid cross – a genetic cross involving individuals both
heterozygous for two traits
– Requires a larger Punnett square because an individual in a
dihybrid cross can produce four possible gametes
Punnett Squares Example Part 1
Ex) In pea plants, there are two alleles for seed shape. The
dominant allele is for round seeds (R), and the recessive allele is
for yellow pods (g). If two pea plants heterozygous for both
characters were crossed, what would be the expected phenotypic
ratios of the offspring for these two characters?
Step 1
Draw and label Punnett square with parent genotypes.
Punnett Squares Example Part 2
Ex) In pea plants, there are two alleles for seed shape. The
dominant allele is for round seeds (R), and the recessive allele is
for yellow pods (g). If two pea plants heterozygous for both
characters were crossed, what would be the expected phenotypic
ratios of the offspring for these two characters?
Step 2
Use parent genotypes to find each box’s genotype.
Step 3
Determine genotypic percentages and ratios.
1 GGRR : 2 GGRr : 1 GGrr :
2 GgRR : 4 GgRr : 2 Ggrr :
1 ggRR : 2 ggRr : 1 ggrr
Punnett Squares Example Part 3
Ex) In pea plants, there are two alleles for seed shape. The
dominant allele is for round seeds (R), and the recessive allele is
for yellow pods (g). If two pea plants heterozygous for both
characters were crossed, what would be the expected phenotypic
ratios of the offspring for these two characters?
Step 4
Use dominance to determine phenotype in each box.
Punnett Squares Example Part 5
Ex) In pea plants, there are two alleles for seed shape. The
dominant allele is for round seeds (R), and the recessive allele is
for yellow pods (g). If two pea plants heterozygous for both
characters were crossed, what would be the expected phenotypic
ratios of the offspring for these two characters?
Step 5
Determine phenotypic percentages and ratios.
9 green pods and round seeds :
3 green pods and wrinkled seeds :
3 yellow pods and round seeds :
1 yellow pods and wrinkled seeds
9:3:3:1
Does the result make sense? Yes.