Final Concepts for Chapter 9Mendelian Genetics

•Allele•Dominant•Recessive•P-generation•F1 generation•F2 generation•Law of independent assortment•Law of segregation•Chromosomes•Pure breed•Trait

•Codominance•Complete dominance•Dihybrid cross•Genotype•Genotypic ratio•Heterozygous•Homozygous•Incomplete dominance•Monohybrid cross•Phenotype•Phenotypic ratio•Probability•Punnett square•Testcross

•Expected/predicted results

•Actual/observed results

•Karyotype

•Amniocentesis

•Linked genes

•Sex-linked disorders

•Autosomal disorders

Independent Assortment vs. Linked Genes

• Mendel did not know about chromosomes when he proposed the Law of Independent Assortment.

• The pea traits he studied happened to be located on different chromosomes – so they did assort independently.

Independent Assortment vs. Linked Genes

• Question: How many traits do you have?

• Question: How many chromosomes (per cell) do you have?

• Question: Is it possible to have only one trait per chromosome? – No, lots of genes are carried or linked

together on the same chromosome.

Independent Assortment vs. Linked Genes

• Do the punnett square for the following cross – assume independent assortment.

Cross two heterozygous tall, heterozygous red flowered plants

T=tall R=red flower

t= short r = white flower

Independent Assortment vs. Linked Genes

TtRr x TtRr

TTRR TTRr TtRR TtRr

TTRr TTrr TtRr Ttrr

TtRR TtRr ttRR ttRr

TtRr Ttrr ttRr ttrr

TR

TR

tr

tR

Tr

Tr tR tr

What is the phenotypic ratio?

Independent Assortment vs. Linked Genes

9:3:3:1 ratio

9 = tall and red

3 = tall and white

3 = short and red

1 = short and white

PROBABILITY:

From this cross, 48 offspring were produced.

1. How many offspring would you expect to be tall and red?

2. How many would expect to be tall and white?

3. How many would you expect to be short and white?

Independent Assortment vs. Linked Genes

Now, do the following cross BUT the genes for tallness and red flowers are linked.

Cross two heterozygous tall, heterozygous red flowered plants

T=tall R=red flowert= short r = white flower

Independent Assortment vs. Linked Genes

• Hint

T t

R r

TtRr X TtRr

Is it possible to produce a Tr gamete?

Independent Assortment vs. Linked Genes

TTRR TtRr

TrRr ttrr

TtRr X TtRr

TR

tr

TR

tr

What is the phenotypic ratio?

3:1

3 = Tall and Red

1 = Short and white

Independent Assortment vs. Linked Genes

• So… out of the 48 offspring, if the genes are linked, how many would be – 1. tall and red?– 2. tall and white?– 3. short and red?– 4. short and white?Answer:tall and red = 36 tall/white = 0short and white = 12 short/red = 0

EXPECTED RESULTS!

Independent Assortment vs. Linked Genes

Is it possible for our Actual Results to show any flowers that are tall/white or short/red?

Yes – how?

Crossing over

Crossing over occurs in meiosis

Pieces of the chromosomes actual switch places.

Complete vs Incomplete Dominance

Codominance – the alleles are equally dominant

Roan Cow Human Blood Type

Sex-linked Traits• Traits carried on the X chromosome

Fill in the genotypes on the pedigree.

Autosomal disorders

• Disorders carried on non-sex chromosomes (first 22 pairs)

• Some are autosomal dominant– Huntington’s disease

• Most are autosomal recessive– Sickle-cell anemia– Cystic fibrosis

Question: How do you know if the pure bred dog you just paid big bucks for is actually pure?

GG? Gg?

Test Cross

• Cross using a homozygous recessive individual with a dominant individual to determine if the dominant individual is heterozygous or homozygous dominant (pure)

• Why use a homozygous recessive individual?

Test Cross

• All offspring produced should show the dominant characteristics if the dominant parent is pure (GG) for the trait.