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Mendelian Genetics.

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Presentation on theme: "Mendelian Genetics."— Presentation transcript:

1 Mendelian Genetics

2 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

3 Mendel chose to study one character at a time.
Character=feature (like flower color); Trait = variant of a character (purple, white)

4 Each cell has 2 genes for a character
One gene for a character is on each chromosome in a homologous pair. One chromosome /gene comes from the mother, while other chromosome /gene comes from the father. Flower Color Genes

5 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

7 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

8 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

9 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.

10 Mendel’s 1st 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 P p

11 A problem in simple dominance…

12 Two alleles: T = tall; t = short P generation: TT x tt gametes
F1 generation: Tt What is the F1 generation’s phenotype? T T t t

13 The grid is called a PUNNETT SQUARE.
Mendel’s mathematician friend, Punnett, showed him how to set up a grid to find the offspring… t t Tt T The grid is called a PUNNETT SQUARE.

14 Some basic nomenclature…
P = parent generation F1 = 1st filial generation (sons and daughters of parents) F2 = 2nd filial generation (cross brother & sister to produce grandchildren)

15 Crossing our F1 generation… Tt x Tt
Genotypes: TT Tt 1 tt Ratio= 1:2:1 Phenotypes: Tall 1 short Ratio= 3:1 T t TT Tt tt T t

16 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%

17 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.

18 unknown genotype with a homozygous recessive
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.

19 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.

20 P TTBB x ttbb TB tb F TtBb All tall, brown eyes

F TtBb x TtBb Tb TB Tb TB tB tb tB tb Each pair of alleles segregated into gametes independent of the other pair. This is Mendel’s 2nd Law LAW OF INDEPENDENT ASSORTMENT

22 Let’s look at why this happens…
B b 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?

23 And the answer is….YES! If both pairs of alleles are on the same pair of homologous chromosome T b t B Genes on the same chromosome are said to be LINKED GENES They are inherited together (red hair/freckles)

24 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.

25 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

26 RR rr Rr RR Rr rr

27 And with Roan cattle… No, there are no pink cows…

28 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

29 Genotype Phenotype AA Type A blood AO Type A blood AB Type AB blood BB Type B blood BO Type B blood OO Type O blood

30 P AO x BO AB AO BO OO gametes A O B O B O A O
What is probability offspring will be Type A blood?

31 The ABO blood groups used to be used in cases of disputed paternity.
DNA fingerprinting has replaced ABO blood grouping in these cases.

32 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.)

33 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?

34 XX XY XX XY X Y What is the probability of having a son? X
Who determines the child’s sex? XX XY X

35 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.

36 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

37 Monosomy – one chromosome instead of a pair
Types of Aneuploidy Monosomy – one chromosome instead of a pair Trisomy – three chromosomes instead of a pair


39 Turner Syndrome genotype = XO no Barr body live female
1 in 5000 births only viable monosomy sterile and short in height most normal IQ

40 Klinefelter Syndrome genotype XXY live male 1 in 2000 births
testes small; breasts enlarged sterile normal IQ

41 YO YO has no X chromosome, so nonliving Not Viable

42 XXX …a Super Female 1 in every 1000 tall, high IQ Amazon women?

43 Down Syndrome… Trisomy 21
3 chromosome 21 1 in 700 in US severely alters phenotype short stature heart defects facial features mental retardation

44 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

45 Red/Green Color Blindness
recessive allele (c) carried on X chromosome XCXC and XCXc have color vision XCY has color vision XcXc and XcY are color blind

46 P XCXc x XCY XCXC XCY XCXc XcY gametes XC Xc XC Y Phenotypes:
1 normal  1 carrier  1 normal  1 colorblind  XC Y XC Xc XCXC XCY XCXc XcY Carrier appears “normal” but carries/passes on recessive allele

47 Hemophilia Hemophilia is sex-linked recessive allele.
Hemophiliacs lack ability to clot blood.

48 Sex-Influenced Disorders
carried on autosomes influenced by sex hormones examples: peptic ulcer male pattern baldness

49 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

50 Epistasis Gene at one locus alters phenotype expressed by gene at another locus. black/brown coat color in mice

51 Polygenic Traits More than one gene affects phenotype.
Example: skin color in humans


53 Karyotype photo of chromosomes arranged in order from largest to smallest can detect: # chromosomes missing pieces moved pieces sex of baby

54 Where do you get cells to do a karyotype?
white blood cells from blood cells from baby from amniocentesis chorionic villus sample (CVS)

55 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


57 Fetal Testing Amniocentesis Chorionic Villus Sampling (CVS)
Carrier recognition (Sickle-cell, Tay-Sachs, etc) Newborn screening (Phenylketonuria)

58 Genetic Disorders Dominant Disorders (Huntington’s disease)
Recessive Disorders (Cystic fibrosis, Tay-Sachs) Sex-linked Disorders (Duchene’s Muscular Dystrophy; Fragile X syndrome Multifactorial Disorders (Heart disease, diabetes, cancer)

59 Phenylketonuria is a recessive genetic disease that causes retardation
Phenylketonuria is a recessive genetic disease that causes retardation. It can be controlled with diet. A “heel stick” is done at birth to detect PKU.

60 Chromosomal Mutations
Deletion (whole or part of chromosome) Inversion (piece breaks off/reattaches wrong) Translocation (piece breaks off/moves to another chromosome)



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