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 gene



May deviate (do NOT follow) from simple Mendelian patterns



Examples



Incomplete dominance, codominance, multiple alleles, pleiotropy

The Spectrum of Dominance



Complete dominance

 

Occurs 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)

P Generation Red

C R C R

 Gametes

C R C W

Pink

C R C W

F 1 Generation Gametes

1 ⁄ 2

C R

1 ⁄ 2

C R

Figure 14.10

F 2 Generation Eggs

1 ⁄ 2

C R

1 ⁄ 2

C R

1 ⁄ 2

C R

Sperm

C R C R C R C W

1 ⁄ 2

C w C R C W C W C W

White

C W C W

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”

Codominance



2 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 Alleles



A type of codominance



Most genes exist in populations



In more than two allelic forms that influence gene’s phenotype



Ex: Human Blood type

The ABO blood group in humans Is determined by multiple alleles Table 14.2

Multiple Alleles (Codominance) Blood Type A B AB O Genotypes I A I A , or I A i I B I B , or I B i I A I B 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 I A I B



50% chance I A i (A type); 50% chance I B i (B type) What are the possible blood types of a child who's parents are both heterozygous for "B" blood type?

I B i X I B i



50% chance I B i, 25% chance I B I B , 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?

I A ? x I A I B



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? I A i and I B i 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= I A I B



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 pattern



Examples:



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

Polygenic Inheritance



AaBbCc AaBbCc

20 ⁄ 64

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

15 ⁄ 64 6 ⁄ 64 1 ⁄ 64



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; need HIGHERpH

Chapter 11 (Part 4)

Human Genetics Honors Genetics Ms. Gaynor

Many human traits follow Mendelian patterns of inheritance



Humans are not convenient subjects for genetic research



However, the study of human genetics continues to advance



We use pedigrees !

Pedigree Analysis



A pedigree



Is a

family tree

that describes the interrelationships of parents and children across generations

Inheritance patterns of particular traits can be traced and described using pedigrees

Ww ww ww Ww Ww ww ww Ww Ww ww WW

or

Ww ww

First generation (grandparents)

Ff Ff ff Ff

Second generation (parents plus aunts and uncles)

FF

or

Ff Ff ff Ff Ff ff

Third generation (two sisters)

ff FF

or

Ff

Widow’s peak

Figure 14.14 A, B (a) Dominant trait (widow’s peak)

No Widow’s peak Attached earlobe Free earlobe

(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 manner



Show up only in individuals homozygous for the alleles



Carriers



Are 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 Symptoms



Physical weakness, pain, organ damage, and even paralysis

Dominantly Inherited Disorders



Some human disorders



Are due to dominant alleles



Example is achondroplasia



Form of dwarfism



lethal when homozygous for the dominant allele

Another Dominant Disorder



Huntington’s disease (HD)

 

degenerative disease of nervous system No 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 21



trisomy 21

Genetic Testing and Counseling



Genetic counselors



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

Tests for Identifying Carriers



For a growing number of diseases



Tests are available that identify carriers and help define the odds more accurately



Examples



Tay Sachs & CF

Fetal Testing



In amniocentesis



The liquid that bathes fetus is removed & tested



In chorionic villus sampling (CVS)



A sample of the placenta is removed and tested

Can make karyotypes, too!

Newborn Screening



Some genetic disorders can be detected at birth



Simple 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 Linkage Honors 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 chromosome

 

Has 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 Morgan



Provided convincing evidence that chromosomes are the location of Mendel’s heritable alleles

Sex linkage explained

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

Thomas Hunt Morgan (Columbia University 1910) Fruit Flies (Drosophila)

melanogaster) © 2007 Paul Billiet ODWS

Morgan’s Choice of Experimental Organism



Morgan worked with fruit flies



Lots 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 w + WILDTYPE w MUTANT

The case of the white eyed mutant Character

Eye color type)

Traits

Red eye (wild White eye (mutant)

P Phenotypes

Wild type (red-eyed) female x White-eyed male

F 1

Phenotypes All red-eyed

Red eye is dominant to white eye

Hypothesis A cross between the F should give us: 3 red eye : 1 white eye 1 flies F 2 Phenotypes Numbers

Red eye 3470 82% White eye 782 18%

So far so good

F 2 An interesting observation The F 2 generation showed the 3:1 red: white eye ratio, but only males had white eyes Phenotypes

Red eyed males Red eyed females White eyed males

White eyed females Numbers

1011 2459 782

0

24% 58% 18%

0%

A reciprocal cross

Morgan tried the cross the other way round white-eyed female x red-eyed male Result All red-eyed females and all white eyed males This confirmed what Morgan suspected The 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 sex



Is an inherited phenotype determined by the presence or absence of certain chromosomes



XX = girl



XY = 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)

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 sons



X Y (Father)



X X (daughter)



X Y (Father)



X Y (son) Mothers alleles to BOTH sons and daughters



can pass sex-linked



X X (Mother)



X X (daughter)



X X (Mother)



X Y (son)

Sex Linkage



If sex-linked recessive on X n



Females have to be X n X n sex-linked trait to show



X n X Females do NOT show sex linked trait



Males have to be X n 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

http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm

Color blind simulation

Another X-Linked Trait = Duchenne Muscular Dystrophy