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Mendel’s Legacy. Genetics is everywhere these days – and it will continue as a dominant force in biology and society for decades to come. . Wouldn’t it be nice if people understood it better?. The Fundamental Question.

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slide1

Mendel’s Legacy

Genetics is everywhere these days – and it will continue as a dominant force in biology and society for decades to come.

Wouldn’t it be nice if people understood it better?

slide2

The Fundamental Question

What is the relationship between genes (genotype) and observable characteristics (phenotype)?

The answer?

Phenotype = Genotype + Environment.

slide3

Genes and Environment Determine Characters

Genetically identical hydrangeas growing in soils of different acidity (different environments).

The phenotype = genotype + environment principle applies equally to human traits.

slide4

Dangerously Ahead of the Game - A Eugenics Exhibit at the 1920 Kansas State Fair

Can history be repeated?

slide5

A Mendelian Genetic Primer

Genes come in pairs that separate in the formation of gametes.

The members of the pair may be identical (homozygous) or non-identical (heterozygous).

Each form of a particular gene is an allele.

slide6

A Mendelian Genetic Primer

Only two alleles of a given gene are possible in an individual although many alleles of a gene are possible within a population.

One allele is dominant over another (or so Mendel believed).

slide9

The Reality of “Round and Wrinkled” – Two Alternative Traits of the Seed Shape Character

Note that each of seed is a new individual of a different generation – seeds are not of the same generation as the plant that bears them.

slide10

A Punnett square, something we’ll cover in a moment.

Mendel’s Monohybrid Cross – P to F1

slide11

Staying the Course – Mendel Continued Crosses to the F2 (the grandchildren)

What was learned?

The green trait was not lost or altered, even though it disappeared in the F1.

One trait is dominant to the other in its expression.

The reappearance of the recessive trait in ¼ of the F2, suggests genes come in pairs that separate in the formation of sex cells.

slide12

Monohybrid Crosses and the Principle of Segregation

A cross between individuals differing in single character is a monohybrid cross.

The analysis of monohybrid crosses allowed Mendel to deduce the Principle of Segregation ....

Genes come in pairs that separate in the formation of sex cells (and these sex cells unite randomly at fertilization).

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Segregation

Principle of Segregation Demystified

The principle of segregation is explained by the behavior of homologous chromosomes at meiosis.

slide14

A Punnett Square is a Handy Way of Analyzing Crosses

In a Punnett square for a monohybrid cross, the Principle of Segregation is applied.

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Characters investigated by Mendel

Consistency is Good

No matter what the character, Mendel observed a 3:1 ratio of characters in the F2.

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Monohybrid Crosses Yielded Consistent Results

Therefore, the Principle of Segregation indeed is a general principle of genetics.

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What Works for Peas Also Works for Humans

In the cross Aa x Aa, where A is a dominant allele for wild type (standard) pigmentation and a is a recessive allele for no pigmentation (albinism), ¾ of offspring will be wild type and ¼ will be albino.

An albino woman

do this monohybrid cross
Do this monohybrid cross
  • In pea plants, round seeds are dominant to wrinkled seeds. If a heterozygous round seed plant is crossed with a heterozygous round seed plant, what is the expected phenotypic and genotypic ratios in the F1 ?
answer
Answer

P – Rr x Rr

Alleles produced for gametes are R and r from each parent

R r

R

r

Phenotypic Ratio: 3 round : 1 wrinkled

Genotypic ratio: 1 RR : 2 Rr : 1 rr

try this one
Try This one !

Fruit fly wing length is controlled by a dominant allele for long wings (L). If a heterozygous long winged fly is mated to a homozygous long winged fly, what is the expected phenotypic and genotypic ratio in their offspring?

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