chapter 13 (part 3) non-mendelian genetics honors genetics ms. gaynor

Chapter 13 (Part 3)

Non-Mendelian Genetics

Honors Genetics

Ms. Gaynor

Extending Mendelian Genetics for a Single Gene

The inheritance of characters by a single geneMay deviate (do NOT follow) from simple Mendelian patternsExamples

Incomplete dominance, codominance, multiple alleles, pleiotropy

The Spectrum of Dominance

Complete dominanceOccurs when the phenotypes of the heterozygote (Hh) and dominant homozygote (HH) are identical

Demonstrates or follows “Mendelian Genetics” inheritance pattern

“Non-Mendelian Genetics”

Incomplete (intermediate) Dominance 1 allele is not completely dominant

over the other, so heterozygote (Hh) has intermediate (or mixed) phenotype between 2 alleles

(like snapdragon flowers)

Figure 14.10

P Generation

F1 Generation

F2 Generation

RedCRCR

Gametes CR CW

WhiteCWCW

PinkCRCW

Sperm

CR

CR

CR

Cw

CR

CRGametes

1⁄2 1⁄2

1⁄2

1⁄2

1⁄2

Eggs1⁄2

CR CR CR CW

CW CWCR CW

Let’s do some practice problems…

Assume incomplete dominance… A red gummy bear mates with a yellow gummy

bear. Red (R) is dominant. What are the genotype/phenotype ratios of their F1 offspring?

100% Rr 100% orange If 2 F1 gummy bears from the question above

mate. What are the genotype/phenotype ratios of their F2 offspring?

25% RR 50% Rr 25% rr 25% Red 50% orange 25% yellow

“Non-Mendelian Genetics”

Codominance2 dominant alleles affect phenotype in separate, distinguishable ways

BOTH phenotypes are present

Ex’s of codominance Some flowers and Roan animals

(cattle & horses)

Roan Animals Show Codominance

Let’s do some practice problems…

Assume codominance… A blue flower mates with a yellow flower. Blue (B)

is dominant. What are the genotype/phenotype ratios of their F1 offspring?

BB= blue Bb= blue & yellow bb= yellow 100% Bb 100% Blue AND yellow flowers If 2 F1 flowers from the question above mate.

What are the genotype/phenotype ratios of their F2 offspring?

25% BB 50% Bb 25% bb 25% Blue 50% blue AND yellow 25% yellow

Multiple AllelesA type of codominanceMost genes exist in populations

In more than two allelic forms that influence gene’s phenotypeEx: Human Blood type

The ABO blood group in humansIs determined by multiple alleles

Table 14.2

Multiple Alleles (Codominance)

Blood Type

Genotypes

A IAIA, or IAi

B IBIB, orIBi

AB IAIB

O ii

Blood Type Practice

A woman with Type O blood and a man, who is Type AB, are expecting a child.  What are the possible blood types of their child? 

ii x IAIB 50% chance IAi (A type); 50% chance IBi (B type)

What are the possible blood types of a child who's parents are both heterozygous for "B" blood type? 

IBi X IBi

50% chance IBi, 25% chance IBIB, 25% chance ii• 75% chance of B type and 25% chance of O type

More Blood Type Practice What are the chances of a woman with Type AB and a man with

Type A having a child with Type O?

IA? x IAIB 0% chance of Type O b/c mom can’t donate “i” allele

Jill is blood Type O.  She has two older brothers with blood types & B.  What are the genotypes of her parents?

IAi and IBi

Jerry Springer did a test to determine the biological father of child The child's blood Type is A and the mother's is B.  Daddy Drama #1 has a blood type of O & Daddy Drama #2 has blood type AB.  Which man is the biological father? 

Dad #1 = ii and Dad #2= IAIB

It has to be Daddy #2

Polygenic Inheritance Many genes (2+) determine one (1) phenotype Many human traits

Vary in the population along a continuum Few genes actually follow a simple Mendelian inheritance

patternExamples:

Height, eye color, intelligence, body build and skin color

Polygenic Inheritance

AaBbCc AaBbCc

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

20⁄64

15⁄64

6⁄64

1⁄64

Fra

cti o

n o

f p

rog

en

y

Nature and Nurture: The Environmental Impact on

Phenotype Departs from simple Mendelian

genetics phenotype depends on environment as well as on genotype

Called multifactorial inheritance Ex: human fingerprints hydrangea flowers

Al in soil; need LOW pH

Add P to soil; needHIGHERpH

Chapter 11 (Part 4)

Human GeneticsHonors Genetics

Ms. Gaynor

Many human traits follow Mendelian patterns of inheritance

Humans are not convenient subjects for genetic researchHowever, the study of human genetics continues to advance

We use pedigrees!

Pedigree Analysis

A pedigreeIs a family tree that describes the interrelationships of parents and children across generations

Inheritance patterns of particular traitscan be traced and described using pedigrees

Figure 14.14 A, B

Ww ww ww Ww

wwWwWwwwwwWw

WWor

Ww

ww

First generation(grandparents)

Second generation(parents plus aunts

and uncles)

Thirdgeneration

(two sisters)

Ff Ff ff Ff

ffFfFfffFfFF or Ff

ff FForFf

Widow’s peak No Widow’s peak Attached earlobe Free earlobe

(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

Pedigrees

Can also be used to make predictions about future offspring

Recessively Inherited Disorders

Many genetic disorders are inherited in recessive mannerShow up only in individuals homozygous for the alleles

CarriersAre heterozygous individuals, who carry recessive allele

but are show “normal” phenotype

Cystic Fibrosis Example of recessive disorder Affect mostly people of

European descent Symptoms

Mucus buildup in the some internal organs

Abnormal absorption of nutrients in the small intestine

Sickle-Cell Disease Another recessive disorder

Affects one out of 400 African-Americans

Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

SymptomsPhysical weakness, pain,

organ damage, and even paralysis

Dominantly Inherited Disorders

Some human disordersAre due to dominant alleles

Example is achondroplasiaForm of dwarfism lethal when

homozygous for the dominant allele

Another Dominant Disorder

Huntington’s disease (HD)degenerative disease of nervous

systemNo obvious phenotypic effects until

about 35 to 40 years of age

HD

Normal

Down Syndrome Down syndrome

Is usually the result of an extra chromosome 21trisomy 21

Genetic Testing and Counseling

Genetic counselorsCan provide information to prospective parents concerned about a family history for a specific disease

Tests for Identifying Carriers

For a growing number of diseasesTests are available that identify carriers and help define the odds more accurately

ExamplesTay Sachs & CF

Fetal TestingIn amniocentesis

The liquid that bathes fetus is removed & tested

In chorionic villus sampling (CVS)A sample of the placenta is removed and testedCan make karyotypes, too!

Newborn Screening Some genetic disorders can be

detected at birthSimple tests are now routinely performed in most hospitals in the United States

Example- PKU test

Chapter 13(PART 5)

The Chromosomal Basis of Inheritance

Introduction to Sex LinkageHonors Genetics

Ms. Gaynor

Gene Linkage Linked genes

Usually inherited together because located near each other on the SAME chromosome

Genes closer together on the same chromosome are more often inherited together

Each chromosomeHas 100’s or 1000’s of genes

Sex-linked genes exhibit unique patterns of inheritance; genes on the X or Y chromosome

Morgan’s Experimental Evidence

Thomas Hunt MorganProvided convincing evidence that chromosomes are the location of Mendel’s heritable alleles

Sex linkage explained Thomas Hunt Morgan

(Columbia University 1910) Fruit Flies (Drosophila)

melanogaster)

http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html

© 2007 Paul Billiet ODWS

Morgan’s Choice of Experimental Organism

Morgan worked with fruit fliesLots of offspring A new generation can be bred every two weeks

They have only 5 pairs of chromosomes

Morgan and Fruit Flies Morgan first observed and noted

Wild type (most common) phenotypes that were common in the fly populations

Traits alternative to the wild type are called mutant phenotypes

WILDTYPE

MUTANT

w+ w

The case of the white-eyed mutant

Character TraitsEye color Red eye (wild type)

White eye (mutant)

P PhenotypesWild type (red-eyed) female x White-eyed male

F1 Phenotypes All red-eyed

Red eye is dominant to white eye

Hypothesis

A cross between the F1 flies should give us: 3 red eye : 1 white eye

F2 Phenotypes Red eye White eye

Numbers 3470

82%

782

18%

So far so good

An interesting observation

F2

Phenotypes Red-eyed males

Red-eyed

females

White-eyed males

White-eyed

females

Numbers 1011 2459 782 0

24% 58% 18% 0%

The F2 generation showed the 3:1 red: white eye ratio, but only males had white eyes

A reciprocal cross

Morgan tried the cross the other way round

white-eyed female x red-eyed maleResult

All red-eyed females and all white-eyed males

This confirmed what Morgan suspectedThe gene for eye color is linked to the X

chromosome

Morgan’s Discovery: Sex Linked Traits

Eye color is linked on X Chromosome

Females carry 2 copies of gene; males have only 1 copy

If mutant allele is recessive, white eyed female has the trait on both X’s

White eyed male can not hide the trait since he has only one X.

The Chromosomal Basis of Sex

An organism’s sexIs an inherited phenotype

determined by the presence or absence of certain chromosomes

XX = girlXY = boy

Inheritance of Sex-Linked Genes

The sex chromosomes Have genes for many

characters unrelated to sex (especially the X chromosome)

A gene located on either sex chromosome Is called a sex-linked gene

(Usually on X chromosome)(Usually on X chromosome)

What genes are on the X chromosome?

carries a couple thousand genes but few, if any, of these have anything to do directly with sex determination

Larger and more active than Y chromosome

What genes are on the Y chromosome?

Gene called SRY triggers testis development, which determines male sex characteristics

This gene is turned “on” ~6 weeks into the development of a male embryo

Y-Chromosome-linked diseases are rare

Sex-linked genes follow specific patterns of

inheritance

Fathers pass sex-linked alleles to ALL their daughters but NONE to their sonsXY (Father) XX (daughter)XY (Father) XY (son)

Mothers can pass sex-linked alleles to BOTH sons and daughters XX (Mother) XX (daughter)XX (Mother) XY (son)

Sex Linkage If sex-linked recessive on Xn

Females have to be Xn Xn to show sex-linked trait

Xn X Females do NOT show sex-linked trait

Males have to be Xn Y to show sex-linked trait

**Most sex-linked disorders affect males; sometimes females

Sex-Linked Disorders Some recessive alleles found on

the X chromosome in humans cause certain types of disorders Color blindness Duchenne muscular dystrophy Hemophilia Male pattern baldness

X-Linked Trait = Male Pattern Baldness

Baldness

Another X-Linked Trait = Hemophilia

About 85% of hemophiliacs suffer from classic hemophilia 1 male in 10 000 cannot produce factor VIII

The rest show Christmas disease where they can’t make factor IX

The genes for both forms of hemophilia are sex linked

Hemophiliacs have trouble clotting their blood

Another X-Linked Trait = Red-Green Colorblindness

Normal vision Color blind simulationhttp://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm

Another X-Linked Trait = Duchenne Muscular Dystrophy