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. –Select Symbols the alleles: –Write the cross: –Gametes each parent can produce: –Make a punnet square. F1: –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. –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. The Symbols (letters you will use) ______________________ The Cross (the parent’s genotypes)_________________________ The Punnett square: