Genetics

Title :

Genetics

Author :

Kim Foglia & Kelly Reidell

Number of slides:

31

Slideshow :

Content transcript :

Genetics

&

The Work of Mendel

Gregor Mendel

* Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas

* used experimental method

* used quantitative analysis

* collected data & counted them

* excellent example of scientific method

Mendel’s work

* Bred pea plants

* cross-pollinate 

true breeding parents (P)

* P = parental

* raised seed & then 

observed traits

* F = filial

* allowed offspring 

to self-pollinate 

& observed next 

generation (F2)

Looking closer at Mendel’s work

What did Mendel’s findings mean?

* Traits come in alternative versions

* purple vs. white flower color

* alleles

* different alleles vary in the sequence of nucleotides at the specific locus of a gene

* some difference in sequence of A, T, C, G

Traits are inherited as discrete units

* For each characteristic, an organism inherits 2 alleles, 1 from each parent

* diploid organism 

* inherits 2 sets of chromosomes, 

1 from each parent

* homologous chromosomes

* like having 2 editions of encyclopedia

* Encyclopedia Britannica 

* Encyclopedia Americana

What did Mendel’s findings mean?

* Some traits mask others 

* purple & white flower colors are separate traits that do not blend 

* purple x white ? light purple

* purple masked white

* dominant allele 

* functional protein

* masks other alleles 

* recessive allele 

* allele makes a 

malfunctioning protein

Genotype vs. phenotype

* Difference between how an organism “looks” & its genetics

* phenotype 

* description of an organism’s trait

* the “physical”

* genotype 

* description of an organism’s genetic makeup

Making crosses

* Can represent alleles as letters

* flower color alleles ? P or p

* true-breeding purple-flower peas ? PP

* true-breeding white-flower peas ? pp

Looking closer at Mendel’s work

Punnett squares

Pp   x   Pp

Genotypes 

* Homozygous = same alleles = PP, pp

* Heterozygous = different alleles = Pp

Phenotype vs. genotype

* 2 organisms can have the same phenotype but have different genotypes

Test cross

* Breed the dominant phenotype —

the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele

How does a Test cross work?

Mendel’s 1st law of heredity

* Law of segregation 

* during meiosis, alleles segregate

* homologous chromosomes separate

* each allele for a trait is packaged into a separate gamete

Law of Segregation

* Which stage of  meiosis creates the 

law of segregation?

Monohybrid cross

* Some of Mendel’s experiments followed the inheritance of single characters 

* flower color

* seed color 

* monohybrid crosses

Dihybrid cross

* Other of Mendel’s experiments followed the inheritance of 2 different characters 

* seed color and 

seed shape

* dihybrid crosses

Dihybrid cross

What’s going on here?

* If genes are on different chromosomes…

* how do they assort in the gametes?

* together or independently?

Is this the way it works?

Dihybrid cross

Mendel’s 2nd law of heredity

* Law of independent assortment

* different loci (genes) separate into gametes independently 

* non-homologous chromosomes align independently

* classes of gametes produced in equal amounts

* YR = Yr = yR = yr

* only true for genes on separate chromosomes or 

on same chromosome but so far apart that crossing over happens frequently

Law of Independent Assortment

The chromosomal 

basis of Mendel’s 

laws…

Trace the genetic 

events through 

meiosis, gamete 

formation & 

fertilization to offspring

Review: Mendel’s laws of heredity 

* Law of segregation

* monohybrid cross 

* single trait

* each allele segregates into separate gametes

* established by Metaphase 1

* Law of independent assortment

* dihybrid (or more) cross

* 2 or more traits 

* genes on separate chromosomes 

assort into gametes independently

* established by Metaphase 1

Mendel chose peas wisely

* Pea plants are good for genetic research

* available in many varieties with distinct heritable features with different variations

* flower color, seed color, seed shape, etc.

* Mendel had strict control over 

which plants mated with which

* each pea plant has male & female 

structures

* pea plants can self-fertilize

* Mendel could also cross-pollinate 

plants: moving pollen from one plant 

to another

Mendel chose peas luckily

* Pea plants are good for genetic research

* relatively simple genetically

* most characters are controlled by a single gene with each gene having only 2 alleles, 

* one completely dominant over 

the other