Presentation on theme: "Mendelian Genetics. Gregor Mendel… Father of Genetics Augustinian monk worked with garden peas studied pea’s characteristics formulated 2 basic."— Presentation transcript:
Gregor Mendel… Father of Genetics Augustinian monk worked with garden peas studied pea’s characteristics formulated 2 basic laws of genetics governing nature of inheritance 50 yrs before we knew of chromosomes; 100 yrs before genes and DNA
Mendel chose to study one character at a time. Character=feature (like flower color); Trait = variant of a character (purple, white)
Each cell has 2 genes for a character One gene for a character is on each chromosome in a homologous pair. Flower Color Genes One chromosome /gene comes from the mother, while other chromosome /gene comes from the father.
The variants (different genes for the same character) are called alleles. Flower Color Genes Purple and white are the 2 alleles (variants) for flower color in peas. Mendel found these alternative genes are responsible for variations in inherited characters.
Some alleles always show themselves…they are said to be Dominant Alleles Other alleles only show themselves if they are found in pairs…they are Recessive Alleles This is called the PRINCIPLE OF DOMINANCE
Dominant genes are shown as a capital letter (P) Recessive genes are shown as a small letter (p) This gives 3 possible genotypes… PP Pp pp
PP is known as pure purple or homozygous dominant Pp is known as hybrid or heterozygous pp is known as pure white or homozygous recessive
The physical appearance that results from this pair of genes is called the PHENOTYPE PP and Pp both have at least one dominant allele so they produce purple flowers. pp has two recessive alleles so they produce white flowers.
Mendel’s 1 st Law The Law of Segregation The two alleles for a character segregate randomly into gametes. Each gamete contains one allele. Remember…Gametes are haploid. Pp
A problem in simple dominance…
Two alleles: T = tall; t = short P generation: TT x tt gametes F1 generation: Tt What is the F1 generation’s phenotype? TTtt
Mendel’s mathematician friend, Punnett, showed him how to set up a grid to find the offspring… Tt t t T The grid is called a PUNNETT SQUARE.
Some basic nomenclature… P = parent generation F1 = 1 st filial generation (sons and daughters of parents) F2 = 2 nd filial generation (cross brother & sister to produce grandchildren)
Crossing our F1 generation… Tt x Tt TTTt tt T t TtTt Genotypes: 1TT 2Tt 1 tt Ratio= 1:2:1 Phenotypes: 3Tall 1 short Ratio= 3:1
Probability Looking at this example… What is the probability that a given offspring will be tall? (3 of the 4 are tall, or ¾) so probability is 75%
Test Cross Note that both TT and Tt have the same phenotype – tall. The genotype of a tall individual, therefore is unknown. We can find out the unknown genotype by performing a test cross.
A test cross crosses an unknown genotype with a homozygous recessive T_ x tt If offspring are all tall, unknown in TT If some are short, unknown is Tt.
Dihybrid Cross Dihybrid crosses study 2 characters… heighth (T=tall; t=short) eye color (B=brown; b=blue) Note we have 2 different sets of alleles.
P TTBB x ttbb TB tb F1 TtBb All tall, brown eyes
F1 TtBb x TtBb Tb TB tBtb tB TB Each pair of alleles segregated into gametes independent of the other pair. This is Mendel’s 2 nd Law LAW OF INDEPENDENT ASSORTMENT
Let’s look at why this happens… Tt Bb Each character’s genes are on a different pair of homologous chromosomes, so each pair assorts independent of the other pair. Is there any time when this would not be true?
And the answer is….YES! If both pairs of alleles are on the same pair of homologous chromosome TbTb tBtB Genes on the same chromosome are said to be LINKED GENES They are inherited together (red hair/freckles)
How many different gametes? AaBBCcDDEeff 2 · 1 · 2 · 1 · 2 · 1 Multiply the # of different alleles in each pair to get the answer. Answer= 8 combinations (Examples: ABcDef, aBCDEf, etc.) Note: each gamete has one of each pair.
Incomplete Dominance (Blending or Partial Dominance) Some alleles aren’t completely dominant, they are partially dominant. Example: Flower color in snapdragons R = red (incomplete dominant) r = white
RR rr Rr RR Rrrr
And with Roan cattle… No, there are no pink cows…
Multiple Alleles Some characters have more than 2 alleles…they have multiple alleles. The ABO blood groups are an example of multiple alleles…the alleles are A = codominant B = codominant O = recessive
Genotype Phenotype AAType A blood AO Type A blood AB Type AB blood BB Type B blood BOType B blood OOType O blood
P AO x BO AOBOgametes ABAO BOOO B O AOAO What is probability offspring will be Type A blood?
The ABO blood groups used to be used in cases of disputed paternity. DNA fingerprinting has replaced ABO blood grouping in these cases.
Autosomes & Sex Chromosomes As you recall, humans have 23 pairs of chromosomes in each cell. 22 pairs are autosomes 1 pair is sex chromosomes (They also have a chromosome in their mitochondria in each cell.)
Sex Chromosomes There are two sex chromosomes… X – required for life Y – determines sex Note that the X and Y are not homologous. Do you know why?
XX XY XXXY XXXY XXXX What is the probability of having a son? Who determines the child’s sex?
In somatic (body) cells of females, one of the two X chromosomes is inactivated. It coils up in the corner of nucleus. It is called a Barr body.
Nondisjunction and Aneuploidy Things can go wrong during meiosis and a gamete can wind up with more than one of a pair of chromosomes or with no chromosome from a pair. This is abnormal segregation is called nondisjunction The abnormal # of chromosomes it produces is called aneuploidy
Types of Aneuploidy Monosomy – one chromosome instead of a pair Trisomy – three chromosomes instead of a pair
Turner Syndrome genotype = XO no Barr body live female 1 in 5000 births only viable monosomy sterile and short in height most normal IQ
Klinefelter Syndrome genotype XXY live male 1 in 2000 births testes small; breasts enlarged sterile normal IQ
YO YO has no X chromosome, so nonliving Not Viable
XXX …a Super Female 1 in every 1000 tall, high IQ Amazon women?
Down Syndrome… Trisomy 21 3 chromosome 21 1 in 700 in US severely alters phenotype short stature heart defects facial features mental retardation
Sex-Linked Disorders Sex-linked disorders are due to genes carried on the X chromosome. They are not found on Y chromosome. Hemophilia Color Blindness
Red/Green Color Blindness recessive allele (c) carried on X chromosome X C X C and X C X c have color vision X C Y has color vision X c X c and X c Y are color blind
P X C X c x X C Y gametes XCXC XCXC XcXc Y XCXCXCXC XCYXCY XCXcXCXc XcYXcY X C Y XCXcXCXc Phenotypes: 1 normal 1 carrier 1 normal 1 colorblind Carrier appears “normal” but carries/passes on recessive allele
Hemophilia Hemophilia is sex-linked recessive allele. Hemophiliacs lack ability to clot blood.
Sex-Influenced Disorders carried on autosomes influenced by sex hormones examples: peptic ulcer male pattern baldness
Pleiotropy one allele having more than one effect (2 phenotypes) example: white tiger also cross-eyed Sickle-cell anemia Red blood cells sickle shape and hemoglobin defective
Epistasis Gene at one locus alters phenotype expressed by gene at another locus. black/brown coat color in mice
Polygenic Traits More than one gene affects phenotype. Example: skin color in humans
Karyotype photo of chromosomes arranged in order from largest to smallest can detect: # chromosomes missing pieces moved pieces sex of baby
Where do you get cells to do a karyotype? white blood cells from blood cells from baby from amniocentesis cells from baby from chorionic villus sample (CVS)
Amniocentesis removes some baby cells in amniotic fluid allow cells to do mitosis (2 wks) add colchicine to stop in metaphase put cells in a hypotonic solution so they burst photograph chromosomes