6.3 Mendel and Heredity

KEY CONCEPT 6.3, 6.4, & 6.5 Mendel’s research showed that traits are inherited as discrete (individual) units.

6.3 Mendel and Heredity

Mendel laid the groundwork for genetics.

• Traits are distinguishing characteristics that are inherited. (eye color, hair color)

• Genetics is the study of biological inheritance patterns and variation.

• Gregor Mendel showed that traits are inherited as discrete units. – Like different colored

marbles mixed together that can still be picked out separately.

6.3 Mendel and Heredity

• Mendel crossed many pea plants and observed these seven traits. His results led to important conclusions (more on that later)

6.3 Mendel and Heredity

The same gene can have many versions.

• A gene is a piece of DNA that directs a cell to make a certain protein.

• Each gene has a locus, aspecific position on a pair ofhomologous chromosomes.

6.3 Mendel and Heredity• An allele is any alternative form of a gene occurring at a

specific locus on a chromosome. (gene=pea shape, alleles= wrinkled or smooth)– Each parent donates

one allele for every gene.

-Genotype: The genetic makeup of a specific set of genes -Phenotype: The physical characteristics of an individual – what you actually SEE

-Homozygous(purebred) describes two alleles that are the same at a specific locus. Ex: (RR or rr)

– Heterozygous (Hybrid) describes two alleles that are different at a specific locus.Ex: (Rr)

6.3 Mendel and Heredity

• Alleles can be represented using letters.

– A dominant allele is expressed as a phenotype (visible trait) when at least one allele is dominant.

– A recessive allele is expressed as a phenotype (visible trait) only when two copies are present.

– Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters.

https://www.youtube.com/watch?v=i-0rSv6oxSY

6.3 Mendel and Heredity

• Mendel used pollen to fertilize selected pea plants (Ex.: Purple Flowers X White Flowers)

Mendel controlled thefertilization of his pea plantsby removing the male parts,or stamens.

He then fertilized the femalepart, or pistil, with pollen froma different pea plant.

– He crossed the P (parent) generation to produce F1 (first filial or first set of offspring) generation.

– He interrupted the self-pollination process in the plants by removing male flower parts.

6.3 Mendel and Heredity

• Skip to: Mendel then allowed the resulting plants to self-pollinate.– Among the F1 generation, all plants had purple flowers

– this is the phenotype (describes physical traits, what we can see)

– F1 plants are all heterozygous this is the genotype (describes the internal makeup of the genes, what we cannot see).

6.3 Mendel and Heredity

6.3 Mendel and Heredity

Back to: Punnett squares illustrate genetic crosses.

• The Punnett square is a grid system for predicting all possible genotypes resulting from a cross.– The axes represent

the possible gametesof each parent.

– The boxes show thepossible genotypesof the offspring.

• The Punnett square yields the ratio of possible genotypes and phenotypes.

6.3 Mendel and Heredity

A monohybrid cross involves one trait.

• Monohybrid crosses examine the inheritance of only one specific trait. (let’s do this one from Mendel’s experiment as an example).

6.3 Mendel and Heredity

• Phenotype (Offspring): 100% Purple flowers

• Genotype (Offspring): 100% Heterozygous

6.3 Mendel and Heredity

• Among the F2 generation (when the F1 generation is crossed with themselves), some plants had purple flowers and some had white

• Let’s do this cross and see how he got these results.

6.3 Mendel and Heredity– F2 Results: (Answer)– Phenotype: 75% Purple, 25% White (3:1 Purple:White)– Genotype: 25% Homozygous Dominant (FF)

– 50% Heterozygous (Ff)– 25% Homozygous Recessive (ff) – Ratio: 1:2:1 Homo Dom: Hetero: Homo Rec– ¼ Ho Do, ¼ Ho Rec, 2/4 Hetero

6.3 Mendel and Heredity

• Practice: Ff x ff• What words would you use to describe the P generation

(parents)?• What would the phenotype and genotype be of the F1

generation?

Phenotype: 50% Purple, 50% White

Genotype: 50% Heterozygous, 50% Homozygous Recessive

6.3 Mendel and Heredity

• Both homozygous dominant and heterozygous genotypes yield a dominant phenotype– Ex: FF and Ff are both purple

• The only combination that shows the recessive phenotype is when homozygous recessive is the genotype – Ex: ff is white

6.3 Mendel and Heredity

• Mendel drew three important conclusions.

1. Traits are inherited as discrete units.

2. Organisms inherit two copies of each gene, one from each parent.

3. The two copies segregateduring gamete formation.–The last two conclusions arecalled the law of segregation.

purple white

6.3 Mendel and Heredity

• http://ed.ted.com/lessons/how-mendel-s-pea-plants-helped-us-understand-genetics-hortensia-jimenez-diaz