Mendelian Genetics

Genetics: The Basics• Allele- An alternative form of a gene

• Diploid organisms have one copy on each homologous chromosome

– Represented by letters:• Capital letter = dominant form • Lower case letter = recessive form

• Example= Eye Color– Controlled by 2 alleles – Blue Eyes = bb– Brown eyes= Bb or BB

Dominant allele-fully expressed in the organism's appearance

Recessive allele-no noticeable effect on the organism's appearance

Genetics: The Basics– Heterozygous: Have 2 different forms of the allele

• Example:– Brown Eyes = Bb = heterozygous

– Homozygous: Have 2 of the same forms of the allele• Example:

– Blue Eyes = bb = homozygous recessive– Brown Eyes = BB = homozygous dominant

Genetics: The Basics– Genotype = the genetic makeup of an organism

• Example = BB, Bb, bb

– Phenotype = the physical expression of genes• Example =

– Brown Eyes = phenotype of either the BB or Bb genotype

– Blue Eyes= phenotype of the bb genotype

Remember that phenotype is not necessarily an appearance-It can be things like enzyme production, behavior, etc!!

It is ANY expression of a gene!!

Gregor Mendel

*1843 entered monastery

*1851-53 studied at Univ. of Vienna

*1857 started breeding garden peas

* 1860 started forging data!!

MENDEL'S MAIN QUESTION

Do units of inheritance retain integrity (preserved) or

blend????

Sample Question: If you cross a purple flower with a white flower are these flower colors retained in future crosses or are they blended to form an intermediate color?

Law of Segregation-two alleles for a character are packaged into separate gametes

2 plants crossedSelf-

fertilized

Mendel's findings

1. Alternative version of genes (alleles) account for variations in inherited characters

Purple flowersWhite flowers

Homologous chromosomes

Mendel's findings

2. For each character, an organism inherits two alleles, one from each parent.

Purple flowersWhite flowers

maternal

paternal

Homologous chromosomes

Mendel's findings

3. If two alleles differ, the dominant allele is fully expressed in the organism's appearance.

Purple flowersWhite flowers

recessive

dominant

Mendel's findings

4. The two alleles for each character segregate during gamete production.dominant recessive

PP pp Seed shape

P p Gametes

Punnett SquarePredicts the results of a genetic cross between individuals with known genotypes

Rules for Genetic Problems1. Identify traits (alleles) and assign letters to represent the various traits: capital letters for dominant traits; lower case letters for recessive traits.

3. Draw individual gametes with corresponding letter for trait.

2. Set up parental cross.

7. Set up Punnett square to identify individual genotypes and phenotypes for F2 offspring.

4. Identify F1 offspring phenotype and genotype.

5. Setup F1 cross.

6. Draw individual gametes with corresponding letter for trait.

EXAMPLE: SEED COLORdominant recessive

CC cc

c

C c c

C

C

c

CCC

Cc Cc

cc

3

1

EXAMPLE: POD SHAPEdominant recessive

s

S s s

S

S

s

SSS

Ss Ss

ss

3

1

SS ss

Monohybrid CrossFollows single trait

Test Cross

Breeding a homozygous recessive with a dominant phenotype (unknown genotype) can determine an unknown allele.

In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait,

what fraction of the offspring should have spherical seeds?

Ss Ss

F1 generation, test cross:

What is the genotypic ratio?What is the phenotypic ratio?

The test cross

To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______.

A. y B. Y C. yyD. YY E. Yy

Predicting the results of a test cross A test cross is used to determine if the genotype of

a plant with the dominant phenotype is homozygous or heterozygous. If the unknown is

homozygous, all of the offspring of the test cross have the __________ phenotype. If the unknown is heterozygous, half of the offspring will have the

__________ phenotype.

A. dominant, recessive B. recessive, dominant

•Question: How are two traits inherited? •DIHYBRID CROSS

•Experimental Approach: A cross involving two true-breeding traits.

System: Pea Plants; seed color (Y/y) and seed shape (S/s).

F1 Generation

F1 Generation

1. Each of the male gametes types (SY, Sy, sY, sy) can fuse with each of the female gametes types (SY, Sy, sY, sy).

2. 16 possible combinations of gametes are possible.

3. We will see that there are 9 possible genotypes and 4 possible phenotypes.

4. The two parental phenotypes, and two new phenotypes were obtained.

F1 Generation

Dihybrid CrossFollows two traits

9:3:3:1 RATIO

The phenotypes of two independent traits show a 9:3:3:1 ratio in the F2generation. coat color is indicated by B (brown, dominant) or b (white)tail length is indicated by S (short, dominant) or s (long).

If the children mate with each other, in the F2 generation all combinations of coat color and tail length occur: 9 are brown/short (purple boxes), 3 are white/short (pink boxes), 3 are brown/long (blue boxes) and 1 is white/long (green box).

Dihybrid Cross

In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d)..  If a squash plant true-breeding for white, disk-shaped fruit  is crossed with a plant true-breeding for yellow, sphere-shaped fruit,  what will the phenotypic and genotypic ratios be for:a. the F1 generation?     b. the F2 generation?

   WD  Wd wD wd

 WD  WWDD  WWDd WwDD WwDd

 Wd  WWDd  WWdd WwDd Wwdd

wD WwDD WwDd wwDD wwDd

wd WwDd Wwdd wwDd wwdd

Phenotypic ratios:9/16 will have white, disk-shaped fruit3/16 will have white, sphere-shaped fruit3/16 will have yellow, disk-shaped fruit1/16 will have yellow, sphere-shaped fruitThis is a 9:3:3:1 phenotypic ratio

Genotypic ratios: 1/16 will be homozygous dominant for both traits (WWDD)2/16 will be homozygous dominant for color and heterozygous for shape (WWDd)2/16 will be heterozygous for color and homozygous dominant for shape (WwDD)1/16 will be homozygous dominant for color and homozygous recessive for shape (WWdd)4/16 will be heterozygous for both traits  (WwDd)2/16 will be heteozygous for color and homozygous recessive for shape (Wwdd)1/16 will be homozygous recessive for color and homozygous dominant for shape (wwDD)2/16 will be homozygous recessive for color and heterozygous for shape (wwDd)1/16 will be homozygous recessive for both traits (wwdd)This is a 1:2:2:1:4:2:1:2:1 genotypic ratio

Law of Segregation-Every individual possesses a pair of alleles for any particular trait and that each parent passes a randomly selected copy (allele) of only one of these to its offspring. 

Law of Independent Assortment- Separate genes for separate traits are passed independently of one another from parents to offspring. These allele pairs are then randomly united at fertilization. 

Inheritance that diverges from Mendel's inheritance

GENE INTERACTIONS

The relationship between the genotype and

phenotype is rarely simple.

* Each character is rarely controlled by one gene

*Each gene usually has more than two alleles, with one not always being dominant over the other

Incomplete Dominance

Heterozygotes show a distinct intermediate phenotype, not seen in homozygotes

Not BLENDED

Traits are separable in

further crosses

Most genes have more than two alleles in a population. (IA, IB, I)

In CODOMINANCE, both alleles are expressed and functional, though they may be different.

PleiotrophicMost genes affect more than one phenotypic character.

Pleiotropy:AlbinismA single defect in one of the enzymes catalyzing tyrosine to melanin can affect multiple phenotypic characters, from eye color to skin color to hair color.

Epistasis

A gene at one locus alters the phenotypic expression of a gene at a second locus.

bb with dominant C alleleresults in brown mouse

Polygenic Inheritance

Additive effect of two or more genes on a single phenotypic character.

SKIN COLOR

Controlled by at least 4 different genes

Sex-linked traitsIn humans, 2 of our 46 chromosomes are

classified as sex chromosomes

•Females = XX•Carried on ova

•Males = XY•Carried on sperm

In females, only 1 X chromosome is activeSex linked traits usually aren’t expressed-

In males, their only X chromosome is active•No other X chromosome to block sex linked trait

Sex-linked traitsIn humans, the genes for colorblindness are both located on the X

chromosome with no corresponding gene on the Y.

Strawberries as they would appear to someone who is red/green colorblind.

Sex Linked Traits

• Alleles are expressed on each of the sex chromosomes

• Female: XAXA or XAXa or XaXa

• Male: XAY or XaY

Setting up a punnet square for sex-linked traits:Mom= XAXa Dad = XAY

XA Xa

XA

Y

Mom is carrier, dad does not have x-linked recessive disorder

Mom isn’t carrier, dad has x-linked recessive disorder

Sex Linked Traits• Can a female end up with an X-linked trait????

– Example = Sex-linked baldness • assume that baldness (b) is recessive • Full head-o-hair (B) is dominant

Hemophilia is an X-linked recessive disorder characterized by the inability to properly form blood clots.

.

Y Linked Traits

Recessive Allele Disorders

Achondroplasia

*Form of dwarfism (dominant allele)

*Heterozygous/

Homozygous dominant individuals have dwarf phenotype

*99.99% of population are homozygous recessive

Dominant Allele Disorders

*Heterozygous/

Homozygous dominant individuals have 6 finger phenotype

*399 out of 400 have 5 digits/appendage: homozygous recessive

Dominant Allele Disorders Polydactyly

Pedigree AnalysisInformation about

presence/absence of phenotypic trait is collected from individuals in a family across generations.

Having the past help predict the future

DOMINANT TRAITRECESSIVE TRAIT (Allelic to left column)

Brown eyes PTC taster Widow's Peak  Middigital hair Tongue roller Detached earlobe A and B blood type (codominant)

Pattern baldness (dominant in males)

Blue eyes (more complex, simplified here) PTC non taster Lack Widow’s peak Hairless mid digits Cannot roll tongue Attached earlobe Type O blood type Pattern baldness (recessive in females)

Common Heritable Traits

Common Heritable Traits

Common Heritable Traits

Common Heritable Traits

Common Heritable Traits

Case Study: In Sickness and In Health

Greg and Olga’s Trip to the Genetic Counselor

Work in groups of 3-4Write down answers to turn in

Part 1: Pedigree Construction10 minutes

• What would the pedigree of Greg and Olga’s families look like?

Part 2: Autosomal Dominant Traits10 minutes

• What is an autosome???

• Do autosomal dominant disorders skip generations?

• Could Greg or his mother be a carrier of the gene that causes myotonic dystropy (MD)? Why?

• Is there a possibility that Greg’s aunt or uncle is homozygous for the MD gene? Why?

• Symptoms of MD sometimes don’t show up until after age 50. What is the possibility that Greg’s cousin has inherited the MD gene?

• What is the possibility that Greg and Olga’s children will inherit the MD gene?

Part 3: Autosomal RecessiveTraits10 minutes

• What are the hallmarks of an autosomal recessive trait (list four)?

• What is it about the inheritance pattern of factor VIII deficiency seen in Greg and Olga’s pedigree that point toward it not being an autosomal recessive trait?

Part 4: Sex-Linked Inheritance10 minutes

• What are the characteristics of X-linked inheritance?

• Why does a son never inherit his father’s defective X chromosome?

• What is required for a female to display a sex-linked recessive trait?

• Referring to the pedigree you drew in Part 1, mark the persons who are carriers of the factor VIII deficiency gene.

• What is the chance that Olga carries the gene for factor VIII deficiency? Calculate the probability that she will pass it to her offspring. Will male children be affected in a different way than female children?

• What is the chance that Greg carries the factor VIII gene? Can he pass the gene on to his sons? His daughters? How will each be affected?