Summary
Summary Genetics Chapter 4
- Course
- Institution
- Book
A summary of Genetics Chapter 4 of the book 'Genetics Analysis& Principles'.
[Show more]
Connected book
Book Title:
Author(s):
- Edition:
- ISBN:
- Edition:
More summaries for
-
Summary ISE Genetics Analysis and Principles - Genetics Part 2!!
-
Chapter 1, overview of genetics
-
Complete Test Bank Genetics Analysis and Principles 7th Edition Brooker Questions & Answers with rationales (Chapter 1-29)
Seller
Follow
sammiexj
Reviews received
Content preview
4 Extensions of Medelian inheritance Mendelian inheritance= inheritance pattern that follows Mendel’s laws; involves the transmission of
eukaryotic genes that are located on the chromosomes found within the cell nucleus.
Obey 2 laws: law of segregation+ law of independent assortment.
Simple Mendelian inheritance= involves a simple (1 gene with 2 different alleles), dominant/ recessive
relationship that produces observed ratios in the offspring that readily obey Mendel’s laws.
4.1 Overview of simple inheritance patterns (fig 4.1)
Purpose of examining inheritance patterns:
To predict the outcome of crosses: 2 heterozygous organisms; not always offspring in a 3:1 phenotypic ratio.
To understand how the molecular gene expression can account for an individual’s phenotype .
Understanding the relationship between the molecular gene expression, function of the product
(protein/ RNA)+ trait in which the protein plays a role.
4.2 Dominant and recessive alleles
Recessive mutant alleles often cause a reduction in the amount or function of
the encoded proteins
Wild-type allele= prevalent allele in a natural population (found in >1% of the population).
Typically encodes a protein that is made in the proper amount+ functions normally.
Genetic polymorphism= 2/ more alleles occur in population.
Polymorphic genes: >1 wild-type allele (each allele is found at a frequency of 1% or higher).
Mutant allele= due to altering wild-type allele by mutation (rare in natural populations)-> associated with diseases.
Likely to decrease the amount/ the expression of a functional protein (mutated allele isn’t always defective!)
Recessive allele usually contains a mutation; causes a defect in the synthesis of a fully functional protein.
Often inherited in a recessive fashion; mutant phenotype visible when the individual has 2 of these alleles.
Recessive allele doesn’t affect the phenotype of the heterozygote; masked by a dominant allele.
2 possible explanations for the wild-type phenotype of the heterozygote:
1. 50% of the normal (functional) protein is enough to provide the wild-type phenotype (fig 4.2).
2. Heterozygote may produce more than 50% of the functional protein.
Increased expression of the normal gene (up-regulated); compensates defective allele’s lack of function.
Dominant mutant alleles usually exert their effects in one of three ways
3 mechanisms that account for most dominant mutant alleles (mutant allele dominant over a wild-type allele):
Gain-of-function mutation= changes a gene product so that it gains a new/ abnormal function.
Mutant gene may be overexpressed/ expressed in the wrong cell type.
Dominant-negative mutation= produces an altered gene product that acts antagonistically to the normal
gene product-> shows a dominant inheritance pattern.
Haploinsufficiency= individual only has 1 functional allele (+1 inactive allele); doesn’t produce normal
phenotype-> shows a dominant inheritance pattern-> e.g., polydactyly in humans.
Traits may skip a generation due to incomplete penetrance and vary in their
expressivity
Incomplete penetrance= allele that is expected to cause a particular phenotype doesn’t.
Polydactyly= autosomal dominant trait; affected individual has additional fingers/ toes.
Single copy sufficient to cause this condition.
Dominant allele doesn’t always ‘penetrate into’/ lead to the phenotype of the individual.
Some carry the allele but don’t express it phenotypically.
Measure of penetrance is described at the population level.
If 60% of heterozygotes carrying a dominant allele exhibit the trait allele; trait is 60% penetrant.
At individual level: trait is either present or not.
, Expressivity= degree to which a trait is expressed.
Polydactyly: individual extra digits hands+ feet (high expressivity) vs only on 1 foot (low expressivity).
Incomplete penetrance+ variable expressivity: range of phenotypes often due to:
Environmental influences
Modifier genes (alter the phenotypic effects of other genes)-> ‘genetic background’ of the individuals.
4.3 Environmental effects on gene expression
Norm of reaction= effects of environmental variation on an individual’s traits-> environmental factors can influence
the allele expression; influencing the phenotypes-> examples:
Temperature-sensitive allele= resulting phenotypes depends on the environmental temperature.
Artic fox: changes coat color summer (grayish brown) vs winter (white).
Humans with phenylketonuria (PKU): can't metabolize phenylalanine.
Standard diet (most protein-rich foods): mental impairment, underdevelopment teeth+ stinky urine.
Detected early+ restricted diet (no phenylalanine): remains symptom free (normal phenotype).
Humans can't synthesize vitamin C; due to a mutant GULO gene-> no problem as vitamin C is in our diet!
4.4 Incomplete dominance, overdominance, and
codominance
Incomplete dominance occurs when two alleles produce an intermediate
phenotype (fig 4.5)
Incomplete dominance= heterozygote that carries 2 different alleles exhibits a phenotype that is intermediate to
those of the corresponding homozygous individuals.
4 o’clock plant: P: CR x CW F1: all pink flowers (CRCW) F2: 1:2:1 phenotypic ratio (red: pink: white).
Not the 3:1 ratio observed in simple Mendelian inheritance.
In this case, 50% of the CR protein isn’t sufficient to produce the red phenotype.
50% functional protein can't accomplish the same level of pigment synthesis that 100% protein can.
Our judgement of dominance may depend on the level of examination (fig 4.6)
Trait dominant/ incompletely dominant: may depend on how closely the trait is examined.
Level of visual examination: R dominant to r.
Mendel visually concluded: RR and Rr round peas+ rr wrinkled peas.
Level of starch biosynthesis: R+ r alleles show incomplete dominance.
Microscopic examination reveals: heterozygote round seeds contain an intermediate number of starch
grains compared to homozygote round seeds intermediate amount of functional protein; not enough
to produce as many starch grains as the homozygote (has 2 copies of the R allele).
Overdominance occurs when heterozygotes have greater reproductive success
(fig 4.7)
Overdominance (heterozygote advantage)= heterozygote has greater reproductive success than either of the
corresponding homozygotes.
EXAMPLE OVERDOMINANCE: SICKLE CELL ANEMIA
Autosomal recessive disorder (on a non-sex chromosome).
Two alleles: HbA (normal hemoglobin)+ HbS (abnormal hemoglobin).
Affected individuals (HbSHbS) produce abnormal form of hemoglobin (hemoglobin S); red blood cells deform
into a sickle shape under conditions of low oxygen concentration-> 2 major consequences:
1. Sickling phenomenon greatly shortenes the life span of the red blood cells results in anemia.
2. Abnormal sickled cells clump partial/ complete blocks in capillary circulation.
Localized areas of oxygen depletion pain+ sometimes tissue and organ damage.
Affected individual tend to have a shorter life span than unaffected ones.
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller sammiexj. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $3.20. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
53068 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now