HEREDITY, GENES & DNA

ENDURING UNDERSTANDINGS

* Heredity is the passing of traits from parents to offspring.

* DNA is a double helix made up of nucleotides.

* A gene is a set of instructions for a specific trait.

* Meiosis is the reproduction of sex cells.

Try Human Wheel using Handout Inherited Characteristics (40)

True-breeding plants are self-pollinated and produce offspring exactly like the parent.

Cross-pollination produces a combination or variety of traits from the parents.

Heredity is the passing of traits from parents to offspring.

MENDEL & HEREDITY

Gregor Mendel studied pea plants to see how traits pass from parent to offspring.

Mendel compared the offspring of true-breeding and cross-pollinated plants.

A dominant trait is a trait that appears in the offspring.

A recessive trait is a trait that does not appear in the offspring.

DOMINANT VS RECESSIVE TRAITS

Mendel's Ratios compare the number of dominant traits with the number of recessive traits.

Probability is the mathematical chance that an event or trait will occur.

insert B7 Quicklab (paper copy)

Genetic Variation in the Classroom

A gene is a set of instructions for a specific trait. Offspring inherit 2 sets of genes, 1 from each parent. These combinations of 2 different forms of the same gene are called alleles.

A genotype is the combination of inheritated alleles.

A phenotype is the appearance of a genotype.

GENES, ALLELES & PUNNETT SQUARES

Homozygous alleles have either both dominant or both recessive genes.

HOMOZYGOUS ALLELES

Heterozygous alleles have 1 dominate gene and 1 recessive gene.

HETEROZYGOUS ALLELES

Write the alleles and fill in the Punnett squares for ...

One parent has homozygous dominant alleles, one parent has heterozygous alleles.

Both parents have heterozygous alleles.

C can curl tongue, c cannot curl.

Punnett Squares use parent alleles to construct a diagram showing all possible phenotypes for a particular trait.

C C

C

c

C

C

C

C

Problem

Consider a cross between a pea plant that is heterozygous for round seeds and a pea plant that has wrinkled seed. The allele for round seeds is dominant (R) over that for wrinkled seeds (r). Determine the genotypes of the offspring and indicate both the genotypic and phenotypic ratios.

Let statements: Let R represent the dominant allele for round seeds.Let r represent the recessive allele for wrinkled seeds.

P generation: Rr X rr

Punnett Square:

Activity

1) Monohybrid Problems 3C / Dihybrid Problem 3U

2) Breeding Pigs

Taking it one step further....

Often in genetics the question comes up whether the inheritance of one trait would affect the inheritance of another trait?

For example if flower colour is governed by flower height?

In order to answer such a question we introduce

Dihybrid Crosses- is a cross between two parents that breed true for different

versions of two traits

Lets revisit the example above....Flower colour is governed by gene PFlower height is governed by gene T

Let P represent the dominant allele for pink colourLet p represent the recessive allele for white colour

Let T represent the dominant allele for tall heightLet t represent the recessive allele for short height

If we crossed a homozygous pink & tall flower with a homozygous white & short flower our punnet square would look like this.

PPTT X pptt

Alleles assort independentlyPTPTPTPT

ptptptpt

All F1 generation have the same genotype & phenotype PpTt (Pink & tall).

However, we are still not sure whether the alleles for colour and height were inherited together or if they segregated independently.Further crossing is required. We need F2

PT PT PT PT

pt

pt

pt

pt

PpTt X PpTt

During Meiosis Alleles assort independentlyPTPtpTpt

PTPtpTpt

PT Pt pT pt

PT

Pt

pT

pt

P - pink p - whiteT - tall t - short

Conclusion

Genes that segregate independently in meiosis do not influence each others inheritancePractice Problems #1~3 pg. 136