Mendel and Genetics

Chapter 14

In the 1800s the popular inheritance theory was “blending”--offspring were a mixture of their parents

◦this suggests that organisms will become uniform over time (we know this isn’t true)

Mendel had a “particulate” theory (genes)

◦this was observed through his observations of pea plants

Gregor Mendel

He carefully planned all his breeding experiments, taking careful notes on the results.

his experiments started with true-breeding varieties

◦ then followed the offspring for 2 generations.

(P, F1, and F2)

Mendel’s Peas.

Through thousands of crosses, Mendel’s observations led to 2 fundamental principles of heredity.

◦ Law of Segregation

two alleles separate during gamete formation (meiosis) and end up in different gametes

dominant and recessive alleles

two heterozygous parents crossed always have a phenotypic ratio of 3:1 (Punnett Squares)

◦ Law of Independent Assortment

each pair of alleles segregates independently of each other pair of alleles during meiosis

the chance of inheriting one trait from either parent is separate from all other traits

for typical Medelian inheritance only

Mendelian Inheritance Patterns

Non-Mendelian Inheritance Certain patterns of inheritance are more complex than those

discovered by Mendel (either controlled by one gene or 2+ genes)

When trait is controlled by a single gene...

◦ Complete Dominance--classic Mendelian patterns (strictly dominant or recessive)

◦ Incomplete dominance--neither allele is completely dominant (blending in heterozygous phenotype) flower color

◦ Codominance--two alleles shown independently in heterozygous phenotype

animal coloration

Incomplete Dominance

Codominance

multiple alleles-when a gene for a specific trait has more than two alleles. Results in multiple phenotypes.◦ This usually works in combination with incomplete

or codominance Human ABO blood groups Rabbit Fur Color

pleiotropy--when a gene has multiple phenotypic effects.

◦Single gene affects multiple things in an organism.

◦Most genetic diseases present this way Cystic fibrosis and Sickle Cell anemia

Lethal Genes: a gene that leads to the death of the organism when inherited in homozygous genotype (either dominant or recessive)◦ Dwarfism in humans (dominant allele)◦ Manx cats (recessive)◦ Yellow coat color in mice (dominant)

When a trait is determined by two or more genes...

◦ epistasis-the phenotype at one locus alters the gene at a second locus Interaction of two genes to control a single

phenotype, does not have an additive effect

Might mask another gene, or cause a completely new phenotype

Labrador Retrievers and coat color 2 genes: E (pigment) e (no pigment) ; B (black), b

(brown)

◦ polygenic inheritance--an additive effect of two or more genes on a single phenotypic character Many genes working together to determine a

particular trait

skin color, height, weight, hair color, eye color in humans

When inheritance depends on chromosomes...

◦sex-linked traits--specific traits are carried on the X or Y chromosome. results in some traits

affecting boys more often than girls

X-linked traits: carried

on X chromosome females carriers;

males have trait or not

Colorblindness, baldness, sickle-cell anemia, hemophilia, Duchenne muscular dystrophy all are examples of sex-linked traits.

If a normal-sighted woman whose father was colorblind marries a colorblind man, what percentage of their sons will be colorblind? Daughters?

Sex-linked traits…

Chromosome Number◦ During meiosis chromosomes can fail to split

evenly (nondisjunction) Aneuploidy

◦ Results in severe phenotypic changes in an individual

◦ Diagnosed via karyotype

◦ Down Syndrome (trisomy 21)◦ Klinefelters Sydrome (XXY)◦ Turner Syndrome (X)

Disorders Resulting from Altered Chromosomes

Chromosome Structure◦ Sometimes parts of chromosomes are altered

during cell division or altered due to environment

Deletion: missing piece Duplication: extra piece Inversion: attach upside down in homologous pair, or

within chromosme Translocation: piece joins non-homologous

chromosome

Cri du chat: deletion chromosome 5 Leukemia: translocation (chromosome 9 attaches to

22) “Philadelphia Chromosome” Fragile X: duplication (repeat at end of X)

Good Morning AP Bio!

Today we are going to discuss our last type of inheritance pattern (linked genes)…then practice solving some of those problems.

Reminder: Test corrections are due tomorrow!

You will have time tomorrow to work through and finish your genetics practice problems packet (due Monday)

Remember: crossing over occurs during meiosis, when chromosomes trade alleles◦ Produces “recombinant chromosomes”

Some genes are located very closely on a chromosome, and are usually inherited together.

They are called “linked genes”

Crossing Over… Revisited

linked genes: genes located near each other on the same chromosome are often inherited together

◦genes do not assort independently, so ratio of offspring varies depending on location of genes

result in genetic recombination (offspring with traits different from parents)

This lack of independent assortment indicates the genes are on the same chromosome.

Thomas Morgan and his grad student first discovered linked genes in drosophila (fruit flies).

When crossing a heterozygous wild-type fly (b+b vg+vg) to a black body, vestigial wings (b vg) he discovered allele frequencies that didn’t match the prediction◦ 83% parental types, 17% recombinant types◦ Identified that crossing over had occurred.

http://www.bozemanscience.com/genetic-recombination-gene-mapping/

The recombination frequency (%) is the same as the map units (distance) between genes on a chromosome◦ Less than 50% recombination = same

chromosome We can use this information to map genes Smaller number = closer together

◦ Greater than 50% recombination = different chromosome Not able to map

Mapping a Chromosome

The crossover frequency (recombination) between genes E and F is 6%, between E and G is 10% and between F and G is 4%.

Determine the sequence of genes on the chromosome.

Practice Problem

Environmental Influence◦ Nature vs.

nurture

◦ Expression of traits determined by environmental influences

When inheritance relies on other things…

Nonnuclear Inheritance (mitochondria and chloroplasts)◦ These organelles have their own DNA that

replicates separately from nuclear DNA

◦ Follows non-mendelian inheritance

◦ All your mitochondrial DNA (mDNA) is from your mom! “mitochondrial diseases”—result from mutations in

mDNA

Genomic Imprinting◦ Phenotype depends on if allele is inherited from

mom or dad (autosomal) Allele from either parent is “silenced” by the

presence of other allele Example of epigenetics

◦ Affects very few genes, not common

Chimera◦ Single organism

composed of genetically distinct traits Two genomes, one

organism!

Results from multiple fertilized eggs fusing during development

Used to visually trace traits within human families (helps identify inheritance patterns)◦ Circle= female◦ Square = male

Pedigrees