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.pdfC1 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.pdfConcept 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.
Crossing over (below)
Crossing over event is less likely to happen in between genes close together on the same chromosome.
Parental combination of alleles are more common in gametes than recombinants.
Whenever you see a ratio where BOTH parental genotypes are more common than recombinant genotypes, you KNOW the genes are LINKED.
In the example above, the following ratio reveals that A + B are linked genes:
24 AB : 1 Ab : 1 aB : 24 ab
Exam questions about linked genes typically use this ratio for linked genes:
9 : 1 : 1 : 5
SciPAD
Page 139 - Dihybrid Crosses - No Linkage
Page 140 - Completing a Dihybrid Cross - Step 2
Page 141 - Dihybrid Cross 1 - Peas
Page 142-143 - Dihybrid Cross - Hairy Guinea Pigs
Page 144-145 - Floppy Eared Rabbits
Page 146-147 - Dihybrid Crosses - The Test Cross
Page 148 - Dihybrid Crosses - Linked Genes
Education Perfect
Tasks called
Concept 9: Multiple Alleles & Lethal Alleles
Concept 10: Linked Genes
