Unit 4: Genetics

1
Unit 4 Genetics Heredity

• Chapters 14 and 15
• Biology, 9th Ed
• By Campbell Reece

2
Chapter 14 Mendel the Gene Idea

• Many suggested the
• blending hypothesis
• genetic material from
• parents mixes
• Correct model is the
• particulate hypothesis
• genes are passed to
• offspring in units called
• genes.

3
Gregor Mendel

• Around 1857, Mendel began breeding garden peas to
  study inheritance.
• Used experimental method
• Used quantitative analysis b/c he collected
  data counted peas
• Excellent example of the scientific method

4
Mendels Experiment

• Why peas? Mendel
• noticed many
• variations in peas.
• Control the mating
• of the pea plants
• record the results!
• Traits were distinct!
• Started w/ true-breeding
• plants
• Most traits are controlled by a single gene
  each gene has 2 alleles (one is completely
  dominant to the other)

5
Mendels Work

• Bred pea plants
• Cross pollinated true-breeding parents (P)
• Raised seeds then observed traits (F1)
• Allowed offspring (F1) to cross-pollinate
  observed the next generation (F2)
• P parents
• F filial generation

6
Mendel Collected Data for 7 Traits
7
Overview of Mendelian Genetics

• Character/Gene heritable
• feature i.e., fur color
• Trait variant of a
• character i.e. brown
• fur or white fur
• Allele form of a gene represented by letters
  i.e., B or b
• True-Bred all offspring
• are the same variety
• Hybridization crossing
• of 2 different traits
• P generation parents
• F1 generation first filial
• generation

8
Closer Look at Mendels Work

• P ? purple flowers X white flowers
• F1 ? 100 purple flowers 4 purple0 white
• Self-pollinate
• F2 ? 75 purple 25 white 3 purple1white

9
Leading to the Law of Segregation

• Traits come in alternative versions
• Ex. Purple vs. white flower color
• Alleles ? different alleles vary in the sequence
  of nucleotides (nitrogen bases) at the specific
  locus of a gene
• Purple flower allele white flower allele are 2
  DNA variations at the flower color locus

10
Law of Segregation

• Law of Segregation The alleles for each
  character segregate (separate) during the
  formation of gametes (meiosis).
• When gametes are produced during meiosis,
  homologous chromosomes separate from each other.
• Each allele for a trait segregates (is packaged
  into a separate gamete)

11
Law of Segregation
12
Law of Segregation

• What meiotic event creates the law of
  segregation?
• Between anaphase I and telophase I when the
  homologous chromosomes separate are packaged
  into different cells
• Remember, Mendel didnt even know DNA or genes
  existed!

13
Traits are inherited as discrete units

• For each gene/character, an organism inherits two
  alleles, 1 from each parent
• Diploid organism inherits one set of
  chromosomes from each parent 2 sets of
  chromosomes

14
Law of Dominance

• If the two alleles differ, then the dominant
  allele is fully expressed in the organisms
  appearance the other, the recessive
• allele, has no effect on the organisms
  appearance
• Purple X White Light purple --- NO!!!!
• Purple masked white

15
Genetic Vocabulary

• Punnett Square predicts
• the results of a cross b/w
• individuals of a known genotype
• Homozygous same alleles
• for a character PP or pp
• Heterozygous different alleles
• for a character Pp or pP
• Phenotype physical
• appearance (words) purple or white flowers
• Genotype genetic make-up
• (letters)
• Testcross crossing a homozygous
• recessive to a dominant phenotype
• (unknown genotype)

16
Genotype vs. Phenotype

• 2 organisms can have the same phenotype, but
  different genotypes
• PP homozygous dominant purple flowers
• Pp heterozygous also purple flowers

17
Monohybrid Cross Practice Problems Complete
Dominance

• 1) A homozygous cream colored mouse (dd) is
  crossed with a heterozygous (Dd) dark mouse.
• a. What are the odds that this couple will
• have a cream colored baby?
• b. What are the odds of a dark mouse?
• 2) In sheep, white is due to a dominant gene (W),
  black is due to its
• recessive allele (w). A white ewe mated to a
  white ram produces a black lamb. How does this
  happen? What are the genotypic and phenotypic
  ratios of the offpspring?
• 3) In chickens, yellow legs (Y) are dominant over
  white legs (y). A yellow legged rooster was
  crossed with a white legged hen. Both kinds of
  offspring were produced. What are the genotypes
  of the parents and the offspring?

18
Law of Independent Assortment

• Law of Segregation involves
• 1 character/gene (monohybrid)
• What about two different genes? (dihybrid)
• The two pairs of alleles
• segregate independently
• of each other
• (in Metaphase I of meiosis)
• Law of Independent Assortment

19
Law of Independent Assortment

• Each pair of alleles for each trait
  segregates into gametes independently
• YyRr ? YR, Yr, yR, yr (four gametes formed)

20
Law of Independent Assortment
21
Law of Independent Assortment

• What meiotic event creates the law of independent
  assortment?
• When the homologous chromosomes line up
  independently of each other during metaphase I of
  meiosis

22
Interesting Historical Facts

• While Mendel was acknowledged by his peers as an
  outstanding plant breeder, his revolutionary work
  was overlooked for 34 years.
• Mendel published Experiments on Plant Hybrids
  in 1865. In 1900, 16 years after his death, a
  number of scientists independently rediscovered
  his work.

23
Interesting Historical Facts

• Charles Darwin proposed that evolution by natural
  selection was dependent on variation in the
  population
• Darwin was unable to propose a mechanism for how
  this variation was transmitted.
• The key was Mendels work, and nearly a century
  after Mendel published his findings, historians
  found a copy of Mendels paper in Darwins study.
  He presumably never read it!

24
Probability and Genetics

• Mendels Laws
• A) Segregation
• B) Independent Assortment
• Reflect same laws of probability that apply to
  tossing coins or rolling dice

25
Probability Genetics

• Calculating probability of making a specific
  gamete is just like calculating the probability
  in flipping a coin
• Probability of tossing heads?
• Probability of making a P gamete.
• P P
• Pp 50 or PP 100
• p P

26
Probability Genetics

• Outcome of one toss has no impact on the outcome
  of the next toss
• Probability of tossing heads each time? 50
• Probability of making a P gamete each time
• P
• Pp 50
• p

27
Rule of Multiplication

• Chance that 2 or more independent events will
  occur together
• Probability that 2 coins tossed at the same time
  will land heads up
• ½ X ½ ¼
• Probability of Pp X Pp ? pp
• ½ X ½ ¼

28
Rule of Addition

• Chance that an event can occur 2 or more
  different ways
• Sum of the separate probabilities

Sperm Egg Offspring
P ½ p ½ Pp ¼
p ½ P ½ pP ¼
1/4 1/4 ------ 1/2
29
Calculating Probability

• Pp X Pp ?????

Sperm Egg Offspring
P ½ P ½ PP 1/4
P ½ p ½ p ½ P ½ Pp ¼ pP ¼ ½
p ½ p ½ pp ¼
30
Calculating Dihybrid Probability

• Rule of Multiplication also applies to Dihybrid
  Crosses
• If you have heterozygous parents,YyRr, what is
  the probability of producing yyrr offpspring?
• A) Probability of producing y gamete ½
• B) Probability of producing r gamete ½
• C) Probability of producing yr gamete is
• ½ X ½ ¼
• D) Probability of producing yyrr offspring is
• ¼ X ¼ 1/16

31
Dihybrid Cross Practice Problems

• Cross a pea plant that is heterozygous for purple
  (P) flowers and homozygous dominant for yellow
  (Y) seeds with a plant that is heterozygous for
  purple flowers and homozygous recessive for green
  seeds.

32
Test Cross

• Cross-breed the dominant unknown phenotype with a
  homozygous recessive to determine the identity of
  the unknown allele.
• If parent is PP ? offspring are all purple (Pp)
• If parent is Pp ? offspring are ½ purple (Pp) ½
  white (pp)

33
Test Cross
34
Test Cross Practice Problems

• In Border Collies, black coat (B) is dominant to
  red coat (b).  A breeder has a black male that
  has won numerous awards.  The breeder would like
  to use the dog for breeding if he is purebred or
  BB.  To learn this information, she testcrosses
  him with a red female (bb).  Answer the following
  questions A, B, C, and D.
• A. If the black male is BB, what kind of gamete
  (sperm) can he
• produce?
• B. If the red female is bb, what kind of gamete
  (eggs) can she
• produce?
• C. If the black male is Bb, what kind(s) of
  gametes (sperm) can
• he produce?
• D. If any of the puppies are red, what is the
  father's genotype?

35
Extending Mendelian Genetics

• Mendel worked with a simple system
• A) Peas are genetically simple
• B) Most traits are controlled by a single gene
• C) Each gene has only 2 alleles 1 of which
• is completely dominant to the other
• The relationship b/w genotype and phenotype is
  rarely this simple!!

36
Non-Single Gene Genetics Incomplete Dominance

• Incomplete Dominance
• appearance b/w
• phenotypes of the 2
• parents an
• intermediate/mixture
• Ex. Snapdragons
• RR red flowers
• RR pink flowers
• RR white flowers
• Red flower X White flower ?

37
Non-Single Gene Genetics Co-dominance

• Codominance Two alleles are both dominant to
  each other, so they are both expressed in a
  heterozygote
• Ex. Black White Checkered Chickens and M, N,
  and MN human blood groups
• B Black chicken
• W White chicken
• Black Chicken X White Chicken ?
• BB X WW All BW (Black White Chickens)

38
Incomplete Dominance Co-Dominance Practice
Problems

1. The color of fruit for plant "X" is determined by
   two alleles.  When two plants with orange fruits
   are crossed the following phenotypic ratios are
   present in the offspring 25 red fruit, 50
   orange fruit, 25 yellow fruit.  What are the
   genotypes of the parent orange-fruited plants?
2. Cross a red fruit with an orange fruit and give
   the phenotypic ratio.
3. Cattle can be red (RR all red hairs), white (WW
   all white hairs), or roan (RW red white
   hairs together). Predict the phenotypic ratios of
   offspring when a homozygous white cow is crossed
   with a roan bull. 
4. What should the genotypes phenotypes for parent
   cattle be if a farmer wanted only cattle with red
   fur? 

39
Dominant Alleles

• NOTE Dominant alleles are NOT always more
  common than recessive alleles!!
• Polydactyly dominant alleles
• Only 1 in 400 people are polydactyl
• Most people are homozygous recessive for
  polydactyly

40
Non-Single Gene Genetics Multiple Allele
Problems

• Multiple Alleles more
• than 2 possible alleles
• for a gene
• Ex. Human blood
• types (ABO)
• 3 alleles IA, IB, and I
• IA IB are dominant to the i allele
• IA IB are co-dominant to each other
• Phenotype Genotype
• A IAIA or IAi
• B IBIB or IBi
• AB IAIB
• O ii

41
Human Blood Types
Genotype Phenotype Phenotype Status
IAIA or IAi Type A Type A Oligosaccharides on the surface of RBC --------
IBIB or IBi Type B Type B oligosaccharides on surface of RBC --------
IAIB Type AB Both Type A Type B oligosaccharides on surface of RBC Universal Recipient
ii Type O No oligosaccharides on surface of RBC Universal Donor
42
Blood Compatibility

• Matching compatible blood groups is critical for
  blood transfusions
• A person produces antibodies against foreign
  blood factors ? oligosaccharides
• If a donors blood has an A or B oligosaccharide
  that is FOREIGN to the recipient, antibodies in
  the recipients blood will bind to the foreign
  molecules
• Binding causes the donated blood cells to clump
  together can kill the recipient

43
Multiple Alleles Practice Problems

1. A woman with Type O blood and a man who is Type
   AB have are expecting a child.  What are the
   possible blood types of the kid? 
2. What are the chances of a woman with Type AB and
   a man with Type A having a child with Type O?
3. A test was done to determine the biological
   father of a child.  The child's blood Type is A
   and the mother's is B.  Man 1 has a blood type
   of O man 2 has blood type AB.  Which man is
   the biological father? 

44
Non-Single Gene Genetics Polygenic Inheritance

• Polygenic Inheritance an additive effect of 2
  or more genes on a single phenotypic character
• Ex. human skin color
• and height
• Phenotypes on a continuum
• Skin Color 3 genes
• A, B, C dark skin
• a, b, c light skin
• Alleles have a cumulative effect therefore.
• AaBbCc ? intermediate/medium skin color

45
Nature vs. Nurture

• Phenotype is controlled by both environment and
  genes
• Color of hydrangea
• flowers is influenced
• by the acidity of
• the soil

46
Chi-Square Test

• Test to see if your data supports your hypothesis
• Compare observed vs. expected data
• A) Is variance from expected due to
• random chance?
• B) Is there another factor influencing data?

47
Chromosomal Theory of Inheritance

• Genes have specific locations on chromosomes and
  chromosomes undergo segregation and independent
  assortment

48
Chromosomal Linkage

• Thomas Hunt Morgan
• Drosophilia melanogaster
• Sex Linkage genes located on sex chromosomes
  (pair 23 in humans)
• Linked Genes genes located on the same
  chromosome tend to be inherited together!

49
Morgans Research First Mutant

• 1st to associate a specific gene with a specific
  chromosome
• Fruit flies have 4 pairs of chromosomes
• Wild type (Normal Phenotype) Fly red eyes
• Discovered mutant white-eyed male

50
Morgans Experiment

• P White eyed male X Red Eyed Female
• F1 All Red Eyed Males Females
• F2 3 red 1 white only males had white eyes
• Q How was the possible?
• A The trait was sex-linked!!!

51
Sex-Linked Traits

• Humans other mammals have 2 sex chromosomes ? X
  Y
• 2 X chromosomes female
• X Y male

52
Human Female Karyotype
53
Human Male Karyotype
54
Genes on Sex Chromosomes

• Y Chromosome
• SRY sex-determining region
• Master regulator for maleness
• Turns on genes for production of male hormones
• X Chromosome
• Other traits, rather than sex determination
• Hemophilia
• Colorblindness
• Duchenne Muscular Dystrophy

55
Sex-Linked Traits Summary

• X-Linked
• Follow the X chromosome
• Males get their X from their mother
• Trait is never passed from father to son
• Y-Linked
• Very few traits
• Only 26 genes
• Trait is only passed from father to son
• Females cannot inherit the trait

56
X-Inactivation

• Female mammals inherit two X chromosomes
• One X becomes inactivated during embryonic
  development
• Condenses into a compact object called a Barr Body

57
X Inactivation Barr Bodies Tortoise Shell Cat
58
Sex-Influenced Traits

• Male Pattern Baldness
• autosomal trait influenced by sex hormones
• age effect as well onset after 30 years old
• dominant in males recessive in females
• B_ bald in males bb bald in females

59
Linked Genes

• Genes on the same chromosomes tend to be
  inherited together
• Close Together ? more likely to be inherited
  together
• Far Apart ? More likely to inherited separately
  (behave as if they are on separate chromosomes)
  more likely to cross over in meiosis

60
What is Recombination?

• Occurs when offspring have different combinations
  of traits than the parents
• Parental Types Same genotype as parents
• Recombitants different genotype from parents

61
Chromosomal Basis of Recombination

• Unlinked Genes (genes on different chromosomes)
  have a 50 frequency of recombination
• Linked genes do NOT assort independently b/c they
  are on the same chromosome tend to move
  together through meiosis fertilization
• Why are there recombitants w/ linked genes if
  they do not assort independently?
• B/c crossing over exchanges genes b/w non-sister
  chromatids
• Crossing over b/w homologous chromosomes breaks
  linkages in parent chromosomes to form new,
  recombitant chromosomes.

62
Chromosomal Basis of Recombination

• Notice that crossing over b/w non-sister
  chromatids makes recombitant chromosomes
• These recombitant chromosomes are packaged into
  gametes

63
Production of Recombitant Offspring
64
Genetic Maps

• Crossing Over Genes that DO NOT assort
  independently of each other
• Genetic Maps the further apart two genes are,
  the higher the probability that a crossover will
  occur b/w them therefore, the higher the
  recombination frequency
• I Map Unit 1 recombination frequency

65
Genetic Maps Continued

• Linkage Maps Genetic
• maps based on
• recombination frequencies
• Not a true picture of a
• chromosome and the
• relative distances b/w
• genes
• Shows the sequence
• of genes on a chromosome,
• not an exact location

66
Genomic Imprinting

• Definition parental effect on gene expression
• Identical alleles may have different effects on
  offspring depending on whether they arrive in
  the zygote via the egg or sperm

67
Genomic Imprinting

• Both disorders below are caused by a partial
  deletion of chromosome 15
• Prader-Willi Syndrome
• Mental retardation
• Obesity
• Short stature
• Inherits abnormal chromosome from father
• Angelman Syndrome
• Jerky movements
• Spontaneous laughter
• Motor and/or mental symptoms
• Inherits abnormal chromosome from mother

68
Extranuclear Genes

• Small amounts of DNA are found in mitochondria
  chloroplasts
• This extranuclear DNA is randomly assorted to
  gametes and does not follow simple Mendelian
  rules of inheritance.
• Maternal inheritance is the rule for
  mitochondrial DNA b/c it comes from the cytoplasm
  of the egg/ovum

69
Genetic Diseases/Disorders

• Carried by Genes
• Autosomal Chromosomes 1-22
• Sex-Linked Chromosome 23
• Dominantly Inherited
• Recessively Inherited
• Co-dominance
• Chromosomal Error
• Monosomy
• Trisomy

70
Recessively Inherited Diseases Cystic Fibrosis

• Primarily whites of European descent
• 1 in 2500 births
• 1 in 25 whites is a carrier
• Defective/absent Cl- channels cause high levels
  of Cl- in the body
• Thick sticky mucus coats
• cells
• Build-up of mucus causes
• infections affects
• pancreas, lungs
• digestive tract
• Live until 20s with treatment
• ( 5 yrs. w/o treatment)

71
Cystic Fibrosis
72
Recessively Inherited Diseases Tay Sachs

• Primarily Jews of eastern European (Ashkenazi)
  descent Cajuns
• 1 in 3600 births
• Non-functional enzyme fails to breakdown lipids
  in brain cells
• Symptoms begin a few months after birth
• Seizures, blindness, degeneration of motor and
  mental skills
• Death before 5 years of age

73
Sickle-Cell Anemia Co-dominance Inheritance

• Primarily Africans or of African descent
• 1 of 400 African Americans
• Caused by substitution of a single amino acid in
  hemoglobin
• When oxygen levels are too low, sickle-cell
  hemoglobin crystallizes into long rods
• 2 alleles are co-dominant
• Both normal abnormal hemoglobins are made in
  the heterozygote (Ss)
• Carriers are usually healthy, although some
  suffer some symptoms of sickle-cell disease under
  oxygen stress

74
Sickle-Cell Anemia Inheritance
75
Heterozygote Advantage Sickle-Cell Anemia

• High frequency of heterozygotes is unusual for an
  allele with severe detrimental effects
• May be a selective advantage for being
  heterozygote
• In Africa, where malaria is common.
• Homozygous normal die of malaria
• Homozygous sickle-cell die of sickle cell
• Heterozygote carriers relatively free from both
  malaria sickle cell

76
Dominantly Inherited Diseases

• Only need one copy of the dominant allele to have
  a dominantly inherited disease
• Huntingtons Disease
• Degenerative disease of the nervous system
• Occurs later in life (35-45 years of age)
• Fatal
• Children of a person with Huntingtons
  Diseasewhat is their chance of getting it?
• Carried on chromosome 4

77
Huntingtons Disease
78
Genetic Counseling Testing

• Amniocentesis uses needle
• Chorionic Villi Sampling suction w/ a tube
• Ultrasound
• Fetoscopy
• Newborn Screening blood tests
• Phenylketonuria - PKU
• Pedigrees traces family genes

79
Amniocentesis andChorion Villi Sampling
80
Pedigree Analysis

• Reveals patterns of inheritance
• Square male
• Circle female
• Filled in square/circle person with trait

81
Royal Hemophilia Pedigree