Predicting Inheritance

Concept 6: Monohybrid Crosses, Test Crosses & Pedigree Charts

Success Criteria

I can use Punnett squares to predict the result of monohybrid crosses.
I can identify the expected phenotypic and genotypic ratios of monohybrid crosses using Punnett squares.
I can explain the usefulness of test crosses in determining the genotype of a given individual.
I can use pedigree charts to identify the probable genotype or phenotype of a given individual.

Vocabulary

Energy

Joule

Transformation

Monohybrid Crosses & Punnett Squares

Success Criteria:
I can use Punnett squares to predict the result of monohybrid crosses.
I can identify the expected phenotypic and genotypic ratios of monohybrid crosses using Punnett squares.

Some vocabulary

Before we get started, here's some terminology you need to know when it comes to 'breeding':

Crossing = mating two individuals (x)
Purebred / true bred = homozygous individuals
Hybrid = heterozygous individuals
P (parental) generation
F1 (first filial) generation
F2 (second filial) generation

Monohybrid crosses

A monohybrid cross is when two individuals are crossed, and only one gene is followed/studied in subsequent generations.

E.g. Pea plants are crossed, and the gene/trait for stem height is followed.

Punnett squares

A Punnett square is a diagram for predicting allele combinations of offspring from a cross between parents of known genotypes.

Example 1 below in tall and dwarf plants.

Example 2 below in yellow and green peas.

Example 3 below in blue and brown eyes.

Test Crosses

Success Criteria:
I can explain the usefulness of test crosses in determining the genotype of a given individual.

A test cross is used to determine the genotype of a phenotypically dominant individual with an unknown genotype.

The “mystery” individual is crossed with a homozygous recessive individual. If:

All F1 have a dominant phenotype, the mystery individual was homozygous dominant.
50% of F1 have dominant and 50% have recessive phenotypes, the mystery individual was heterozygous.

Pedigree Charts

Success Criteria:
I can explain the usefulness of test crosses in determining the genotype of a given individual.

It is unethical and impractical to carry out monohybrid and test crosses on humans. Instead, the inheritance of human traits are studied through family trees. A family tree describes the traits of parents and children across generations.

This example follows the recessive albinism mutation in this family.

When a child (Sam) differs from both parents (Karen & Brian), the child must be recessive.

A recessive trait can ‘skip’ a generation; a dominant trait cannot.

C1 Task 1: Watch this video.

M2.5 C6. Genetic Crosses Worksheet.pdf

C1 Task 2: Complete some of these worksheet activities.

SciPAD

Page 123 - Mendel's Peas

Page 124 - Punnett Square Review

Page 125 - Genotypic & Phenotypic Ratio Review

Page 126-127 - Purebred Peas

Page 128 - The Test Cross

Page 129-130 - Pedigree Charts

Page 131 - Pedigree Rats

Education Perfect

Task called 'Concept 6: Genetic Crosses'.

Concept 7: Incomplete Dominance & Codominance

Success Criteria

I can describe incomplete dominance and codominance.
I can solve the genotype and phenotype of individuals from crosses involving incomplete dominance and codominance.

Vocabulary

Energy

Joule

Transformation

Incomplete Dominance

Success Criteria:
I can describe incomplete dominance and codominance.
I can solve the genotype and phenotype of individuals from crosses involving incomplete dominance and codominance.

Neither allele has dominant control over the trait. Neither alleles are fully expressed.

The heterozygote offspring (e.g. Rr flower) is phenotypically intermediate (i.e. blended) between two homozygote parents (RR x rr).

Codominance

Success Criteria:
I can describe incomplete dominance and codominance.
I can solve the genotype and phenotype of individuals from crosses involving incomplete dominance and codominance.

Both alleles in a heterozygous organism (RW) contribute independently and equally (i.e. not blended) to the phenotype.

Cattle coat alleles red (R) and white (W) are codominant, producing a ‘roan-cow’.

SciPAD

Page 133 - Incomplete Dominance - Blue Chickens

Page 134-135 - Codominance

Education Perfect

Task called 'Concept 7: Incomplete Dominance & Codominance'.

Concept 8: Multiple Alleles & Lethal Alleles

Success Criteria

I can describe and solve the genotype and phenotype of individuals from crosses involving multiple alleles.
I can describe and solve the genotype and phenotype of individuals from crosses involving lethal alleles.

Vocabulary

Energy

Joule

Transformation

Multiple Alleles

Success Criteria:
I can describe and solve the genotype and phenotype of individuals from crosses involving multiple alleles.

Multiple alleles are alleles of which there are more than two alternatives available at one locus/for one gene. (So, more than two alleles for one gene).

The presence of a recessive allele has no effect on the blood group in the presence of a dominant allele.

Lethal Alleles

Success Criteria:
I can describe and solve the genotype and phenotype of individuals from crosses involving lethal alleles.

A lethal allele is an allele that produces a phenotypic effect that causes the death of the organism (at any stage of life - embryonic, child, or adult).

Lethal alleles are caused by mutations in a gene essential for growth, development, and survival of an organism.

Recessive lethal alleles
Dominant lethal alleles: Causes death in both homozygote and heterozygote (only need one copy for lethal phenotype). E.g. Huntington’s disease.

Recessive lethal alleles

Recessive lethal alleles cause death in homozygotes only.

Two copies of the lethal allele are required to express the lethal phenotype.

For example: Achondroplasia (dwarfism)

For example: cystic fibrosis

For example: sickle-cell anaemia

Dominant lethal alleles

Dominant lethal alleles cause death in both homozygotes and heterozygotes. Only one copy of the lethal allele is needed to express the lethal phenotype.

For example: Huntington’s disease.

SciPAD

Page 136 - Multiple Alleles

Page 137 - Paternity Problems

Page 138 - Lethal Alleles

Education Perfect

Task called 'Concept 8: Multiple Alleles & Lethal Alleles'.

M2.5 C7-9. Predicting Inheritance Retrieval Grid.pdf

Concept 9: Dihybrid Crosses & Linked Genes

Success Criteria

I can use Punnett squares to predict the result of dihybrid crosses.
I can identify the expected phenotypic and genotypic ratios of dihybrid crosses using Punnett squares.

Vocabulary

Energy

Joule

Transformation

Hei Mahi

Is the Manx allele completely dominant, incompletely dominant, codominant or recessive? Justify your answer.

Dihybrid Crosses

Success Criteria:
I can use Punnett squares to predict the result of dihybrid crosses.
I can identify the expected phenotypic and genotypic ratios of dihybrid crosses using Punnett squares.

A dihybrid cross is mating between two organisms where the inheritance patterns of two genes are studied.

When there is no linkage, these two genes described are carried by separate chromosomes and are sorted independently of each other during meiosis. This results in a greater number of gamete types produced (four types) when two genes are considered.

Dihybrid cross phenotypic ratio of F2 produced by complete dominance:

9 : 3 : 3 : 1

Hei Mahi

Get the following items for today's practical:

--- Four cards that represent two homologous pairs.

--- Four beads that represent paired alleles from two genes.

--- Scissors that represent _______. (Fill in the blank!)

Linked Genes

Success Criteria:
I can explain how linked genes lead to unexpected phenotype ratios.

Linked genes are genes that are located on the same chromosome, and are inherited together.

Independent assortment (right)

Since linked genes they are on the same chromosome, they cannot independently assort. This usually leads to less genetic variation in gametes, for these linked genes.

In contrast, unlinked genes are on different chromosomes, so they can independently assort. This leads to more genetic variation in gametes, for these unlinked genes.