mendelian genetics. genetics: the basics allele- an alternative form of a gene diploid organisms...
Post on 21-Dec-2015
237 views
TRANSCRIPT
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
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.
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).
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
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 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.
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
Pedigree AnalysisInformation about
presence/absence of phenotypic trait is collected from individuals in a family across generations.
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)
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.
.
Polygenic Inheritance
Additive effect of two or more genes on a single phenotypic character.
SKIN COLOR
Controlled by at least 3 different genes
In the following cross between two mulatto genotypes (AaBbCc x AaBbCc), each parent produces eight different types of gametes and these gametes combine with each other in 64 different ways resulting in a total of seven skin colors. The skin colors can be represented by the number of capital letters, ranging from zero (no capital letters) to six (all capital letters). The approximate shades of skin color corresponding to each genotype are shown in Table 1. Note: Skin color may involve at least four pairs of alleles with nine (or more) shades of skin color.
Gametes ABC ABc AbC Abc aBC aBc abC abc
ABC 6 5 5 4 5 4 4 3
ABc 5 4 4 3 4 3 3 2
AbC 5 4 4 3 4 3 3 2
Abc 4 3 3 2 3 2 2 1
aBC 5 4 4 3 4 3 3 2
aBc 4 3 3 2 3 2 2 1
abC 4 3 3 2 3 2 2 1
abc 3 2 2 1 2 1 1 0
SKIN COLOR
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
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?
• What is it about the inheritance pattern of factor VIII deficiency seen in Greg and Olga’s pedigree that point toward it 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?