Mendelian Genetics Honors Biology. Pre-Mendelian Theory of Heredity Blending Theory—hereditary...

Mendelian Genetics

Honors Biology

Pre-Mendelian Theory of Heredity

Blending Theory—hereditary material from each parent mixes in the offspring Individuals of a population should

reach a uniform appearance after many generations

Once traits are blended, they can no longer be separated out to appear in later generations

Pre-Mendelian Theory of Heredity

Problems—inconsistent with observations: Individuals of a population don’t

reach uniform appearance Traits can skip generations

Modern Theory of Heredity

Based on Gregor Mendel’s fundamental principles of heredity Parents pass on discrete inheritable

factors (genes) to their offspring These factors remain as separate

factors from one generation to the next

Useful Genetic Vocabulary

Homozygous—having 2 identical alleles for a given trait (PP or pp)Heterozygous—having 2 different alleles for a trait (Pp); ½ gametes carry one allele (P) and ½ gametes carry the other allele (p)Phenotype—an organism’s expressed traits (purple or white flowers)Genotype—an organism’s genetic makeup (PP, Pp, or pp)

Mendel’s Principles of Heredity

First Law of Genetics: Law of Segregation alternate forms of genes are responsible for

variations in inherited traits for each trait, an organism inherits 2 alleles,

one from each parent If 2 alleles differ, one is fully expressed

(dominant allele); the other is completely masked (recessive allele)

2 alleles for each trait segregate during gamete production

Mendel’s Discoveries

Developed true-breeding lines—populations that always produce offspring with the same traits as the parents when parents are self-fertilized

Counted his results and kept statistical notes on experimental crosses

Crosses Tracking One Characteristic: Flower Color

x

x

x

x

x

x

x

Ratio3.15:1

3.14:1

3.01:1

2.96:1

2.95:1

2.82:1

2.84:1

3:1

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

1

2

1 White

3

1

Purple

Purple

Purple

Genotypic Ratio 1:2:1 Phenotypic Ratio 3:1

Genotype versus Phenotype

The Testcross

The cross of any individual to a homozygous recessive parent

Used to determine if the individual is homozygous dominant or heterozygous

CAUTION:Must perform many, many crosses to be statistically significant

Mendel’s Principles of Heredity

Second Law of Genetics: Law of Independent Assortment During gamete formation, the

segregation of the alleles of one allelic pair is independent of the segregation of another allelic pair

Law discovered by following segregation of 2 genes

Dihybrid Cross

Mendelian Inheritance Reflects Rules of Probability

Rules of Multiplication: The probability that independent events will occur simultaneously is the product of their individual probabilities.

Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive?Answer:

Probability that an egg from the F1 (Pp) will receive a p allele = ½

Probability that a sperm from the F1 will receive a p allele = ½

Overall probability that 2 recessive alleles will unite at fertilization: ½ x ½ = ¼



Question: For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 plant having the genotype YYRR?Answer:

Probability that an egg from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼

Probability that a sperm from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼

Overall probability of an F2 plant having the genotype YYRR: ¼ x ¼ = 1/16

Works for Dihybrid Crosses:


Rules of Addition: The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.


Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will being a heterozygote?Answer:

There are 2 ways in which a heterozygote may be produced: the dominant allele may be in the egg and the recessive allele in the sperm, or the dominant allele may be in the sperm and the recessive allele in the egg.


Probability that the dominant allele will be in the egg with the recessive in the sperm is ½ x ½ = ¼ Probability that the dominant allele will be in the sperm with the recessive in the egg is ½ x ½ = ¼Therefore, the overall probability that a heterozygote offspring will be produced is ¼ + ¼ = ½

Pedigree Analysis

Analysis of existing populationsStudies inheritance of genes in humansUseful when progeny data from several generations is limitedUseful when studying species with a long generation time

= female

= male

= affected individual

= mating

= offspring in birth order I and II are generations

Symbols:

= Identical twins

= Fraternal twins

I

II

Dominant Pedigree:

I

II

III

For dominant traits:•Affected individuals have at least one affected parent•The phenotype generally appears every generation•2 unaffected parents only have unaffected offspring

Recessive Pedigree:

I

II

III

For recessive traits:•Unaffected parents can have affected offspring•Affected progeny are both male and female

Recessive Human Disorders

Sickle-cell anemia; autosomal recessive Caused by single amino acid

substitution in hemoglobin Abnormal hemoglobin packs together to

form rods creating crescent-shaped cells Reduces amount of

oxygen hemoglobin can carry

Genetic Testing & Counseling

Genetic counselors can help determine probability of prospective parents passing on deleterious genes Genetic screening for various known

diseases alleles (gene markers)


Fetal testing

Amniocentesis

needle inserted into uterus and amniotic fluid extracted Test for certain chemicals or proteins

in the fluid that are diagnostic of certain diseases

Karyotype-can see chromosome abnormalities


Fetal testing Chorion Villus Sampling Suctions off a small amount of fetal tissue

from the chorionic villus of placenta Karyotype-can see chromosome

abnormalities

Ultrasound at 12 weeks--can see any physical abnormalities

Variations to Mendel’s First Law of Genetics

Incomplete dominance—pattern of inheritance in which one allele is not completely dominant over the other Heterozygote has a phenotype that is

intermediate between the phenotypes of the 2 homozygous dominant parent and homozygous recessive parent

Incomplete Dominance in Snapdragon Color

Genotypic ratio:

Phenotypic ratio:

1 CRCR: 2 CRCW: 1 CWCW

1 red: 2 pink: 1 white

F2

Variations to Mendel’s First Law of Genetics

Codominance—pattern of inheritance in which both alleles contribute to the phenotype of the heterozygote

Multiple Alleles

Some genes may have more than just 2 alternate forms of a gene.

Example: ABO blood groups A and B refer to 2 genetically determined

polysaccharides (A and B antigens) which are found on the surface of red blood cells (different from MN blood groups) A and B are codominant; O is recessive to A and B

Multiple Alleles for the ABO Blood Groups

3 alleles: IA, IB, i

Pleiotropy

The ability of a single gene to have multiple phenotypic effects (pleiotropic gene affects more than one phenotype) Example: In tigers and Siamese cats, the gene that

controls fur pigmentation also influences the connections between a cat;s eyes and the brain. A defective gene cause both abnormal pigmentation and cross-eye condition

Sickle-cell disease—impact of abnormal hemoglobin can affect other organs

Epistasis

Interaction between 2 nonallelic genes in which one modifies the phenotypic expression of the other. If epistasis occurs between 2 nonallelic genes, the phenotypic ratio resulting from a dihybrid cross will deviate from the 9:3:3:1 Mendelian ratio

CC, Cc = Melanin depositioncc = AlbinismBB, Bb = Black coat colorbb = Brown coat color

A cross between heterozygous black mice for the 2 genes results in a 9:3:4 phenotypic ratio 9 Black (B_C_) 3 Brown (bbC_) 4 Albino (__cc)

Polygenic TraitsSkin pigmentation in humans--3 genes with the dark-skin allele (A, B, C) contribute one “unit” of darkness to the phenotype. These alleles are incompletely dominant over the other alleles (a, b, c)--An AABBCC person would be very dark; an aabbcc person would be very light--An AaBbCc person would have skin of an intermediate shade

Chromosome Theory of Inheritance

Based on Mendel’s observations and genetic studies and cytological evidence Mendelian factors (genes) are located

on chromosomes It is the chromosomes that segregate

and independently assort

Certain genes are linked They tend to be

inheritedtogether because they reside close together onthe same chromosome

Experiment

Explanation: linked genes

PpLI PpLI Long pollen

Observed PredictionPhenotypes offspring (9:3:3:1)

Purple longPurple roundRed longRed round

Parentaldiploid cellPpLI

Most gametes

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

Meiosis

Fertilization

Sperm

284212155

215717124

P I

P L

P L

P L

P LP LP I

P L P I

P I

P L

P I

P I

P I

P I

P L

Purple flower

Figure 9.19

Genes on the same chromosome tend to be inherited together

Crossing over can separate linked alleles

Producing gametes with recombinant chromosomes

A B

a b

Tetrad Crossing over

A B

A b

a b

a B

GametesFigure 9.20 A

Crossing over produces new combinations of alleles

Thomas Hunt Morgan Performed some of the early studies of crossing over using the fruit

fly Drosophila melanogaster

Experiments with Drosophila revealed linkage traits. Why Drosophila?

Easily cultured Prolific breeders Short generation times Only 4 pairs of chromosomes, visible under microscope

Figure 9.20 B

Demonstrated the roleof crossing over in inheritance

Figure 9.20 C

Experiment

Gray body,long wings(wild type)

GgLI

Female

Black body,vestigial wings

ggll

Male

Offspring Gray long

965 944 206 185

Black vestigial Gray vestigial Black long

Parentalphenotypes

Recombinantphenotypes

Recombination frequency = = 0.17 or 17%391 recombinants

2,300 total offspring

Explanation

GgLI(female)

ggll(male)

G L

g l

g l

g l

G L g l G l gL g l

Eggs Sperm

G L g lg l g l g l g l

LglG

Offspring

Morgan’s experiments

Morgan and his students Used crossover data to map genes in

Drosophila

Figure 9.21 A

Geneticists use crossover data to map genes

One of Morgan’s students, Alfred Sturtevant, used crossing over of linked genes to develop a method for constructing a genetic map.This map is an ordered list of the genetic loci along a particular chromosome.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Sturtevant hypothesized that the frequency of recombinant offspring reflected the distances between genes on a chromosome.The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency. The greater the distance between two

genes, the more points between them where crossing over can occur.

Sturtevant used recombination frequencies from fruit fly crosses to map the relative position of genes along chromosomes, a linkage map.


Can be used to map the relative positions of genes on chromosomes.

Figure 9.21 B

Mutant phenotypes

Shortaristae

Blackbody(g)

Cinnabareyes(c)

Vestigialwings(l)

Browneyes

Long aristae(appendageson head)

Gray body(G)

Redeyes(C)

Normalwings(L)

Redeyes

Wild-type phenotypes

Chromosomeg c l

9% 9.5%

17%

Recombinationfrequencies

Figure 9.21 C

Recombination frequencies


Fig. 15.5b

Sturtevant used the testcross design to map the relative position of three fruit fly genes, body color (b), wing size (vg), and eye color (cn). The recombination frequency between cn

and b is 9%. The recombination frequency between cn

and vg is 9.5%. The recombination frequency between b

and vg is 17%. The only possible

arrangement of these three genes places the eye color gene between the other two.


Fig. 15.6

Sturtevant expressed the distance between genes, the recombination frequency, as map units. One map unit (sometimes called a

centimorgan) is equivalent to a 1% recombination frequency.

What is the sequence of these three genes on the chromosome?

A series of matings shows that the recombination frequency between the black-body gene (b) and the gene for short wings (s) is 36%. The recombination frequency between purple eyes (p) and short wings is 41%. The recombination frequency between black-body gene and purple eyes is 6%.

Answer

B 36% SP 41% S

B 6% P

P 6% BB 36% S 6% + 36% = 42%P 41% S

You may notice that the three recombination frequencies in our mapping example are not quite additive: 9% (b-cn) + 9.5% (cn-vg) > 17% (b-vg). This results from multiple crossing over events. A second crossing over “cancels out” the

first and reduces the observed number of recombinant offspring.

Genes father apart (for example, b-vg) are more likely to experience multiple crossing over events.


Some genes on a chromosome are so far apart that a crossover between them is virtually certain.In this case, the frequency of recombination reaches is its maximum value of 50% and the genes act as if found on separate chromosomes and are inherited independently. In fact, several genes studies by Mendel are

located on the same chromosome. For example, seed color and flower color are

far enough apart that linkage is not observed. Plant height and pod shape should show

linkage, but Mendel never reported results of this cross.


•If the recombination frequency is 50% or greater, the genes are not linked•If the recombination frequency is less than 50%, the genes are linked

Genes located far apart on a chromosome are mapped by adding the recombination frequencies between the distant genes and intervening genes.Sturtevant and his colleagues were able to map the linear positions of genes in Drosophila into four groups, one for each chromosome.


Fig. 15.7

A linkage map provides an imperfect picture of a chromosome.Map units indicate relative distance and order, not precise locations of genes. The frequency of crossing over is not actually

uniform over the length of a chromosome.Combined with other methods like chromosomal banding, geneticists can develop cytological maps. These indicated the positions of genes with

respect to chromosomal features.More recent techniques show the absolute distances between gene loci in DNA nucleotides.


SEX CHROMOSOMES AND SEX-LINKED GENES

Chromosomes determine sex in many species

In mammals, a male has one X chromosome and one Y chromosome

And a female has two X chromosomes The Y chromosome

Has genes for the development of testes

The absence of a Y chromosome Allows ovaries to develop

(male) (female)

Parents’diploidcells

Sperm Egg

Offspring(diploid)

44+

XY

44+

XX

22+X

22+Y

22+X

44+

XX

44+

XY

Figure 9.22 A

Other systems of sex determination exist in other animals and plants

22+

XX

22+X

76+

ZW

76+

ZZ

32 16

Figure 9.22 D

Figure 9.22 C

Figure 9.22 B

Sex-linked genes exhibit a unique pattern of inheritance

All genes on the sex chromosomes Are said to be sex-linked

In many organisms The X chromosome carries many genes unrelated to

sex

For genes on X chromosomes, females have 2 copies of gene—can have 2 different alleles

For genes on X chromosomes, males have only one allele; the allele they express

Males’ X comes from mom (dad contributes Y) Males are said to be hemizygous If allele is recessive, it will be expressed

A male receiving a single X-linked allele from his mother

Will have the disorder A female

Has to receive the allele from both parents to be affected

In Drosophila White eye color is a sex-linked trait

Figure 9.23 A

The inheritance pattern of sex-linked genes

Is reflected in females and malesFemale Male

Sperm

Xr Y

XR

Xr Y XR XR

XR Xr XR YEggs

R = red-eye alleler = white-eye allele

Female Male

Sperm

XR Y

XR

XR Y XR Xr

XR XR XR Y

Eggs

Xr Xr XR Xr Y

Female

Sperm

Xr Y

XR

Xr Y XR Xr

XR Xr XR Y

Eggs

Male

Xr Xr Xr Xr Y

Figure 9.23 B Figure 9.23 C Figure 9.23 D

F1 F2

All red eyes All red eyes and

½ red

eyes and

½ white

eyes

CONNECTIONSex-linked disorders affect mostly

males Most sex-linked human disorders

Are due to recessive alleles Are mostly seen in males

Queenvictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

AlexisFigure 9.24 A Figure 9.24 B

The End

Nature versus Nature

Environmental conditions can influence the phenotypic expression of a gene, so that a single genotype may produce a range of phenotypes One may have a history of heart disease in their family and thus be at risk of heart disease themselves. If this person watches his/her diet, exercises, doesn’t smoke, etc. his/her risk of actually developing heart disease decreases

Mendelian Genetics Honors Biology. Pre-Mendelian Theory of Heredity Blending Theory—hereditary...

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