Monohybrid Crosses

True Breeding

• Mendel started his experiments using true breeding pea plants also called purebreds.
• For example tall pea plants always fertilized by tall pea plants produce only tall offspring.
• Tall x tall 🡪 tall
• Short or drawf pea plants always fertilized by tall pea plants produce only tall offspring.
• Short x tall 🡪 tall

Cross Breeding

• Cross breeding occurs when two purebreds cross fertilize
• Cross breeding a tall x short pea plant produces only tall plants because tall is the dominant allele for pea plant height.

Allele Combinations

Each allele combination has a name:

Homozygous Dominant - 2 dominant alleles (YY)

- See the dominant phenotype

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Heterozygous - 2 different alleles (Yy)

-still see the dominant phenotype

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Homozygous Recessive - 2 recessive alleles (yy)

- See the recessive phenotype

Mendel’s Laws

1- Each parent contributes one allele during cross fertilization. If a pure-breeding plant self fertilizes, each offspring receives identical copies of the same allele.

2- Dominant alleles are always expressed when the recessive allele is present.

3- Each pair of alleles segregates during the formation of sex cells.

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Mendel’s Laws

1. Law of Segregation

States that inherited traits are determined by pairs of genes and that each of the genes separates into separate gametes.

2. Law of Independent Assortment

States that inheritance of one trait does not affect the inheritance of alleles for another trait.

Add this slide instead of current mendels law slide

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PARENT and FILIAL GENERATIONS

• The first generation is always called the parent or P1 GENERATION.
• The offspring of the P1 generation are called the first filial or F1 generation.
• The offspring of the F1 generation are called the F2 generation or second filial generation.

Punett Squares

The results of fertilization are shown in a Punett Square

• Step 1

Show the parents alleles

Mother phenotype 🡪 homozygous dominant tall

Genotype 🡪TT

Father phenotype 🡪 homozygous recessive short

Genotype 🡪tt

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F1 generation

Step 2 – complete Punnett square

• Show the possible genotypes for the offspring with the parental gametes

F1 generation

Step 3 – do offspring ratios

• Offspring genotype ratio 🡪 4Tt
• Offspring phenotype ratio 🡪 all tall