Date post: | 16-Jan-2016 |
Category: |
Documents |
Upload: | steven-hines |
View: | 248 times |
Download: | 0 times |
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