Genetics: Mendel and Punnett Squares PPT

Biology Students’ Physical Traits

  • Take a count of the different traits of the people in this class.

  • Why are some traits more common than others?

  • Examples:- Detached Earlobes: Yes/No

    • Widow’s Peak: Yes/No

Genetics

  • Genetics - the study of heredity.

Human Genetic Code

  • The human body has 100 trillion cells.

  • Each cell (except blood cells) contains the human genome, which is all the genetic information necessary to build a human being.

  • Six feet of DNA are packaged into 23 pairs of chromosomes (one from each parent in each pair) in the cell nucleus.

  • Each of the 46 human chromosomes contains the DNA for thousands of individual genes (units of heredity).

  • DNA in each gene contains four chemical bases: adenine (A), thymine (T), guanine (G), and cytosine (C).- A pairs with T, and G pairs with C.

  • Proteins, made of amino acids, are essential ingredients of all organs and chemical activities.

  • Their function depends on their shapes, which are determined by the 50,000 to 100,000 genes in the cell nucleus.

  • Source: National Human Genome Research Institute

12.1 What is the Physical Basis of Inheritance?

  • Inheritance is the process by which the characteristics of individuals are passed to their offspring.

  • Genes encode these characteristics.

Genes

  • A gene is a unit of heredity that encodes information for the form of a particular characteristic.

  • The location of a gene on a chromosome is called its locus.

Alleles

  • Homologous chromosomes carry the same kinds of genes for the same characteristics.

  • Genes for the same characteristic are found at the same loci on both homologous chromosomes.

  • Genes for a characteristic found on homologous chromosomes may not be identical.

  • Alternate versions or forms of genes found at the same gene locus are called alleles.

  • Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes.

  • If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus.

  • If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid).

Examples of Homozygous and Heterozygous

  • Tomato plant with homologous chromosomes.

  • The M locus contains the M gene, which is involved in determining leaf color. Both chromosomes carry the same allele of the M gene. This tomato plant is homozygous for the M gene.

  • The D locus contains the D gene, which is involved in determining plant height. Both chromosomes carry the same allele of the D gene. This tomato plant is homozygous for the D gene.

  • The Bk locus contains the Bk gene, which is involved in determining fruit shape. Each chromosome carries a different allele of the Bk gene. This tomato plant is heterozygous for the Bk gene.

12.2 How Mendel Laid the Foundations of Modern Genetics

  • Who Was Gregor Mendel?

  • Doing It Right: The Secrets of Mendel’s Success

Mendel’s Pea Plants

  • Mendel was born in **** in what is now the Czech Republic.

  • He studied science & mathematics before he became a priest & teacher.

  • Mendel worked as a priest and gardener for the .

  • He worked with **** plants.

Why Pea Plants?

  • _- small in size

  • **- large amount of offspring

  • Have ____ observable “either or” traits

  • Pea plants are also able to reproduce in two ways: self-fertilize and cross-fertilize.

Reproduction in Flowers

  • Flowers have both male and female reproductive parts.

  • Can be:-

    • ___

    • ___

Self-Pollination

  • ___ plants can fertilize themselves.

  • Offspring are to the parent if the plant is a pure or true breed.

  • Mendel let his plants self-pollinate to find the true-breeding plants

Cross-pollination

  • Cross-pollinators are not self-fertilizing- they are fertilized by the _ of another flower

  • produce offspring that are a mix of the parents called

Making Pea Plants Unable to Self-pollinate & then Cross-Pollinating them

  • Remove anthers from one plant.

  • Collect pollen from a different plant.

  • Transfer pollen to the stigma of the individual whose anthers have been removed. Mendel could choose which plants reproduce

Genes & Dominance: 7 "either or" traits

  • Flower Color: Purple, White

  • Flower Position: Axial, Terminal

  • Seed Color: Yellow, Green

  • Seed Shape: Round, Wrinkled

  • Pod Shape: Inflated, Constricted

  • Pod Color: Green, Yellow

  • Stem Length: Tall, Dwarf

12.3 Inheritance of Single Traits

  • The Language of a Genetic Cross

  • Mendel’s Flower Color Experiments

  • Alleles of a Gene Are Dominant or Recessive

  • How Meiosis Separates Genes: Segregation

  • Understanding the Results of Mendel’s Flower Color Experiments

  • “Genetic Bookkeeping”

  • Practical Application: The Test Cross

Generations

  • P= _- parents

  • F1= _- children

  • F2= _- Grand children

  • F3= _- Great grand children

Mendel’s Experiment

  • _ cross

  • P generation = Crossed two true-breeding plants with different traits- Purple x White

  • F1 generation= was not a mix, but resembled one parent- All Purple Phenotype

Conclusions

  • That traits were controlled by _

  • Gene for flower color

  • Genes come in many varieties called alleles- Ex. Purple flower allele (P) or white flower allele (p)

  • Some alleles are dominate and others are recessive- Principle of __

    • If an organism inherits the dominant allele than it will show the dominant trait

    • PP, Pp, pP =purple but pp=white

  • What happens to the recessive? Is it hiding or did it disappear?

Another Experiment

  • Mendel allowed the F1 generation to self-fertilize- F1 generation=- All Purple (dominant phenotype)

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    • F2 generation =- Most plants expressed the dominant trait purple phenotype

      • Some expressed the recessive trait white phenotype

    • How did the recessive separate from the dominant?

The Results of Mendel's F_1 Crosses for Seven Characters in Pea Plants

  • Approximately 3:1 in F_2

The Law of Segregation

  • Body cells contain 2 for a gene.

  • Gametes, sperm & eggs, contain 1 allele for a gene, so each parent can only pass on ½ of their chromosomes

  • Offspring’s cells contain 2 alleles for a gene 1 from each parent

Diploid (2n) vs. Haploid (n)

  • In humans n = 23

  • Human body cells have _ pairs of chromosomes- Diploid = 2 x 23

  • Gametes, sperm and egg, have 23 individual chromosomes- Haploid = 23

  • Offspring are produced when _ and _ donate chromosomes- Diploid = 2 x 23

Phenotype vs. Genotype

  • _ - physical characteristics

  • _ - the genetic makeup of the alleles

  • Homozygous dominant PP

  • Homozygous recessive pp

  • Heterozygous Pp

  • Examples:- Purple Phenotypes-

    • _ – homozygous dominant genotype

      • _ – heterozygous genotype

    • White Phenotype-

      • _ – homozygous recessive genotype

Punnett Square Predicts Probability

  • Lets say that:-

    • Mother- Phenotype: Purple

      • Genotype: PP homozygous

    • Father- Phenotype: Purple

      • Genotype: PP homozygous

    • Offspring- Phenotype: _% Purple

      • Genotype: _% PP homozygous

Punnett Square Predicts Probability

  • Lets say that:-

    • Mother- Phenotype: Purple

      • Genotype: Pp heterozygous

    • Father- Phenotype: Purple

      • Genotype: Pp heterozygous

    • Offspring- Phenotype: ****% purple *% white

      • Genotype: ::____

Punnett Square Predicts Probability

  • Lets say that:-

    • Mother- Phenotype: Purple

      • Genotype: Pp heterozygous

    • Father- Phenotype: White

      • Genotype: pp homozygous

    • Offspring- Phenotype: ****% purple *%white

      • Genotype: :

Punnett Square Predicts Probability

  • Lets say that:-

    • Mother- Phenotype: White

      • Genotype: pp homozygous

    • Father- Phenotype: White

      • Genotype: pp homozygous

    • Offspring- Phenotype: ****% white

      • Genotype: ****% pp homozygous recessive

Punnett Square Explanation

  • Each true-breeding plant of the parental generation has Identical alleles, PP or pp.

  • Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.

  • Union of the parental gametes produces F_1 hybrids having a Pp combination. Because the purple-flower allele Is dominant, all these hybrids have purple flowers.

  • When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.

  • This box, a Punnett square, shows all possible combinations of alleles In offspring that result from an F1 x F1 (Pp x Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm.

  • Random combination of the gametes results In the 3:1 ratio that Mendel observed in the F_2 generation.

What if you don’t know the genotypes?

  • Run a _!

  • Cross an unknown with a known homozygous recessive.

  • Why?-

  • Can use ratios

  • Ex: Cross a purple flower with a white flower and get 100% purple offspring.

  • Purple parent was P__.