Genetics Since MendelUnit 9 – Lesson 2 Notes

Modern Genetics• We know about dominant and recessive

genes, now we are going to learn about other types of genetic traits, such as

– Incomplete dominance– Multiple alleles– Polygenic Inheritance– Mutations

Incomplete Dominance

• Neither allele for a trait is dominant.

• The phenotype produced is intermediate between the two homozygous parents.

Multiple Alleles• More than two alleles are multiple alleles.

• Traits controlled by multiple alleles produce more than three phenotypes.

Multiple AllelesThere are 3 alleles for the gene that determines blood type. (Remember: You have just 2 of the 3 in your genotype --- 1 from mom & 1 from dad).

Notice that, according to the symbols used in the table above, that the allele for "O" (i) is recessive to the alleles for "A" & "B".

ALLELE IA IB i

CODES FOR Type "A" Blood Type "B" Blood Type "O" Blood

GENOTYPES IAIA IAi

RESULTING PHENOTYPES Type A Type A

IBIB IBi

Type B Type B

IAIB Type AB

ii Type O

Polygenic Inheritance• A group of gene pairs acts together to

produce a trait, which creates more variety in phenotypes.

• Many human traits are controlled by polygenic inheritance, such as hair and eye color.

Polygenic Inheritance Example

Mutations• Mutations – genes that are

altered or copied incorrectly.

– A mutation can be harmful, beneficial, or have no effect.

– Chromosome disorders – caused by more or fewer chromosomes than normal.

Mutations– Down syndrome – caused

by an extra copy of chromosome 21.

Recessive Genetic Disorders• Both parents have a recessive allele responsible

for the disorder and pass it on to their child.

• Because the parents are heterozygous, they don’t show any symptoms.

PP Pp

Pp pp

P

P

p

p

Disorder Expressed

Recessive Genetic Disorders Examples

• Homozygous recessive disorders.

– PKU– Cystic Fibrosis– Sickle-Cell Anemia

Other Genetic Disorders• Genes that determine the sex of an organism are XX in

females and XY in males.

• Females produce eggs with an X chromosome only. Males produce sperm with either an X or Y chromosome.

EGGSXX

X

X

X

SPERMS

XX

YY

Y

Other Genetic DisordersDisorders

– Turners Syndrome XO

– Triple X Syndrome XXX

– Klinefelter syndrome XXY

– XYY XYY

Sex-linked Disorders

• An allele inherited on an X or Y chromosome is a sex-linked gene.

• Color blindness is a sex-linked disorder caused by a recessive allele on the X chromosome.

Pedigree• A pedigree follows a trait through generations of a

family.

– Carriers of trait are half shaded.– People with trait are fully shaded.– Squares show males.– Circles show females.

Orange Shows the Trait

Who carries the gene? M or F

How many people are colorblind?