Unit 8: Genetics & HeredityUnit 9: Human Genetic Disorders
Ch. 8: Heredity & Ch. 11: Human Genetics
heredity
• What is genetics?– the study of heredity
• passing of traits from parents to offspring
Unit 8: Genetics & Heredity
Chromosomes in Cells• Remember…
– body cells are diploid• 2 of each chromosome
– 1 from mom & 1 from dad
– gametes (sperm & eggs) are haploid• 1 of each chromosome
– Why?» So zygote gets right # of
chromosomes…
• Why is your combination of genes unique?– Chance. Which sperm will fertilize which egg?
• get ½ your chromosomes from mom & ½ from dad• meiosis (formation of gametes)
– crossing-over during prophase 1– alignment of chromosomes during metaphase 1
Genes
Genes & Alleles• What is a “gene”?
– section of chromosome that codes for a specific protein
• & determines a specific trait (ex. hair color, eye color, ear shape, etc.)
– genes are paired on homologous chromosomes (chromosomes that carry info for same type of trait)• different forms of genes for the
same trait are called “alleles”– ex. brown eyes & blue eyes
Dominant & Recessive Alleles• Each parent contributes 1 allele (form of gene) for
trait & can be dominant or recessive
– What is a dominant allele?• allele that prevents expression of (“masks”/“hides”) recessive
trait
– What is a recessive allele?• allele whose trait can be seen only when the organism is pure
(homozygous) for that trait
Dominant & Recessive Alleles• How are alleles
represented?– with letters
• usually the first letter of the dominant trait
– If the same letter is used for dominant & recessive, how do we know which allele is which?
» CAPITAL = DOMINANT» lowercase = recessive
Allele Combinations• What does “homozygous” mean?
– both alleles are the same• homozygous (pure) dominant (ex. AA)• homozygous (pure) recessive (ex. aa)
• What does “heterozygous” mean?– both alleles are different
• heterozygous (hybrid) (ex. Aa)
Genotype vs. Phenotype• What is “genotype”?
– organism’s actual genetic “code”/make-up (alleles)
• What does the genotype do?– codes for protein that causes
trait (phenotype)
• How do we represent an organism’s genotype?– 2 letters (one for each allele)
• one from mom & one from dad– ex. PP, Pp, pp
Genotype vs. Phenotype• What is “phenotype”?
– the outward (physical) expression of the genotype (trait we “see”)
• What actually causes the “phenotype” (trait) we see?– the protein that is produced (due
to the organism’s genotype “code”/alleles)
• How do we represent an organism’s phenotype?– usually an adjective
• ex. purple, white, tall, short, etc.
Genotype is Expressed as a Phenotype• Ex. Let P = purple & p = white
– homozygous (pure) dominant• genotype PP• phenotype = purple
– homozygous (pure) recessive• genotype pp• phenotype = white
– heterozygous (hybrid)• genotype Pp• phenotype = purple
– dominant trait “masks/hides” recessive trait
Gregor Mendel• Father of Genetics
– 1822-1884• Wondered why cert
ain traits disappear in one generation, yet reappear in the next & tried to determine how traits were passed from parent to offspring.
• studied garden pea plants with 7 different traits with clearly different forms– bred hybrids– applied statistics
• Deduced that consistent ratios of traits in offspring indicated that the plants transmitted distinct “units”
Gregor Mendel
Mendel’s Experiments• What happened when Mendel mated a pure purple
parent (PP) & a pure white parent (pp)?– all F1 offspring:
• purple phenotype• heterozygous (hybrid) genotype
– Pp
• What happened when Mendel let the heterozygous (hybrid) F1 offspring from his first experiment self-pollinate?– So… Pp x Pp (monohybrid cross)
• F2 offspring weren’t all purple… 3 purple : 1 white– Always 3 :1 when both parents are hybrids
Mendel’s Experiments
Mendel’s Principle of Dominance• What did Mendel notice
from his experiments?–white x white always
produced white, but purple x purple produced ~3/4 purple & ~1/4 white• So, purple seemed to mask
white sometimes.– …”dominant” trait prevented
expression of “recessive” trait
» PUPRLE = dominant» white = recessive
Mendel’s Law of Segregation• during gamete
formation, each gene for a trait separates so that each gamete receives only 1 of each gene– happens during
meiosis I when homologous chromosomes line up & separate
Test Cross• Mendel determined two
genotypes resulting in purple flowers with test crosses– Bred dominant phenotype
(unknown genotype) with recessive phenotype (“pure” homozygous since shows recessive trait)
• If get all purple offspring parent = “pure” homozygous PP
• If get ½ purple & ½ white offspring parent = “hybrid” heterozygous Pp
• What are Punnett Squares?– a way to predict the results o
f crosses (mating) • letters outside represent
possible alleles in gametes of each parent
– top = one parent & side = other
• letters inside boxes represent possible allele combinations (genotypes) in offspring (& phenotypes)
Predicting Traits in Offspring
• Punnett Squares can also be used to determine probability & ratios in possible offspring
Predicting Traits in Offspring
BB Bb
Making a Punnett Square• Ex. Parents are Tt & tt genotypes…
– So… Tt x tt is our cross (mating)
Passing Traits to Offspring & Probability• What is probability?
– chance an event will occur• What is the chance of
getting heads? tails?– ½
• If you flip two coins, of getting 2 heads? 2 tails?
– ½ x ½ = 1/4
• What is the chance of a couple having a boy? a girl?
– 1/2
• of having five girls?– ½ x ½ x ½ x ½ x ½ = 1/32
» or ( ½ )5 = 1/32
Passing Traits to Offspring & Ratios
• What is a “genotypic ratio”?– probable ratio of
genotypes (alleles) in offspring of a given cross• Ex. If cross Pp & Pp
– 1PP : 2Pp : 1 pp
Passing Traits to Offspring & Ratios• What is a
“phenotypic ratio”?– probable ratio of
phenotypes (traits) in offspring of a given cross
– resulting from the genotypes of the offspring
• Ex. If cross Pp & Pp• 3 purple : 1 white
Passing Traits to Offspring & Ratios• What is an “expected ratio”?
– ratio we expect to get based on probability (P. Square)
• What is an “observed ratio”?– ratio we actually get
• Why would these be different?– fertilization is random– some embryos die during
early stages
Mendel’s Principle of Independent Assortment• Genes for different traits segregate independently during
gamete formation when they are located on different chromosomes…
BA
b
Ba
b
What if genes are on the same chromosome?
• called “linked”• DO NOT sort independently
Genes on samechromosome
Genes on samechromosome
meiosis
Dihybrid Cross• involves study of inheritance patterns for organisms
differing in 2 traits (each w/ 2 forms).– Mendel used dihybrid cross to determine if different
traits of pea plants, such as flower color & seed shape, were inherited independently.
Dihybrid Cross Animation
• To figure out combination of genes in gametes for two traits that are independently assorted use:– “foil”– (probability) tree diagram
• Ex. Parent w/ AaBb genotype will make the following gametes:– AB– Ab– aB– ab
BA
b
Ba
b
Dihybrid Cross
Dominant/Recessive is Not Always the Method of Inheritance
• Traits are not always as clearly defined as the 7 pea plant traits Mendel studied.– examples of non-dominant/recessive inheritance
• incomplete dominance• codominance• multiple alleles• sex determination• sex-linked traits• polygenic inheritance
– Continuous variation
Incomplete Dominance• No allele is dominant
over another– results in 3 phenotypes –
“dominant” 1, intermediate (mixed), “dominant” 2. • Genotypic & phenotypic
ratios same– 1 CRCR : 2 CRCW : 1 CWCW
– 1 red : 2 pink : 1 white
– Ex. Pink four o’clock flowers
Codominance• What is meant by codominance?
–both alleles “expressed” equally• Ex. Roan cow = mixture of both red & white hairs
Codominance–Ex. human blood types exhibit codominance
(as well as multiple alleles)• A & B are codominant & “expressed” equally
–IA = IB (codominant)– i (recessive)
» So… (IA = IB ) > i
•How many possible genotypes are there?
•How many phenotypes?
•Can you spot the blood type that is the result of codominance?
Multiple Alleles• What is meant by
multiple alleles?– more than 2 different
forms of an allele exist• but individual has just 2
– 1 from mom & 1 from dad
– Ex. human blood types• 3 alleles
– IA (A)– IB (B)– i (o)
•How many possible genotypes are there?
•How many phenotypes?
•Can you spot the blood type that is the result of codominance?
• Agouti rabbits–4 alleles w/ dominance relationships
• Agouti is dominant to chinchilla; both are dominant to Himalayan; all three are dominant to albino… C > cch > ch > c–agouti rabbit (wild type)
» Phenotype: brown, Genotype: CC, Ccch, Cch, Cc
– “Chinchilla” (mutant)» Phenotype: silvery gray, Genotype: cchcch, cchch, cchc
– “Himalayan” (mutant)» Phenotype: white w/ black, Genotype: chch or chc
– “Albino” (mutant)» Phenotype: white, Genotype: cc
Multiple Alleles
Sex Determination• How many chromosomes do humans have (in
somatic cells)?– 46… 23 pairs
• pairs 1 – 22 = autosomes (“body” chromosomes)• 23rd pair determines gender = sex chromosomes
– XX = female– XY = male
What is the probability of having a son? A daughter?
Sex Determination• Which parent’s chromosomes determines if
offspring will be a boy or girl???? Why?– Dad’s
• if he gives X girl• If he gives Y boy
– …mom always gives X• so it can’t be her
What is the probability of having a son? A daughter?
Sex-linked Inheritance• X & Y chromosomes not fully homologous. Why?
– X is bigger & carries more genes
Sex-linked Inheritance• How many alleles will a male have for
traits carried only on the X chromosome?–ONE because only have one “X”
chromosome (Y doesn’t have allele)• What is this called?
–X-linked or sex-linked» Ex. eye color in fruit flies, hemophilia in humans,
colorblindness in humans
Sex-linked Inheritance• X-linked (recessive) traits & disorders are
more common in males. Why???– b/c female has XX, more likely she will have a
copy of dominant allele… males = XY… can only get dominant allele on X (& only have 1 X)• female can be XGXG, XGXg, XgXg
– normal, (normal) carrier, affected• male can only be XGY or XgY
– normal or affected
• How do we make predictions made using Punnett squares for sex-linked traits?– Consider sex chromosome (X/Y) & allele for the
trait it carries (“exponent”) TOGETHER as a unit…• ex. XG (= X w/ dominant allele), Xg (= X w/ recessive
allele), Y (= Y w/ NO allele)
Sex-linked Inheritance
• What if a female is heterozygous (XGXg)?– she does not show the trait/have the disorder,
but is a carrier• & can pass gene to offspring
• Can a male be a carrier?– No, b/c only has one
X chromosome w/ allele… so… either has it or doesn’t
Sex-linked Inheritance
XG Xg
XG
Y
XG XG XG Xg
XG Y Xg Y
• Drosophila (fruit fly) eye color is sex-linked–Let XR = red eye allele, Xr = white eye allele,
Y = no allele• What are sex, phenotype, & genotype of each
offspring? Any carriers for white eye gene?–female w/ red eyes = XRXR
–female w/ red eyes = XRXr » carrier for white eye gene
–male w/ red eyes = XRY–male w/ white eyes = XrY
Sex-linked Inheritance
XR Y
XR
Xr
XR XR XR Y
XR Xr Xr Y
Polygenic Inheritance• What is polygenic
inheritance?–When many genes
affect a single trait• shows range of
phenotypes from one extreme to other (continuous variation)
–Ex. in humans: hair color, height, skin color
Expression of Genes• Genes can interact with one another to
control various other patterns of inheritance– Most characteristics that make up individual’s
phenotype not inherited in Mendelian patterns• Ex. Modifier genes affect eye color
– influence amount, intensity, & distribution of melanin (color pigment) in eye cells
epistasis = phenomenon in which the expression of one gene depends on the presence of one or more 'modifier genes'
• Environment in which organism develops is another factor that affects expression–Probably due to how enzymes (proteins)
operate at different temperature• Higher temps may “deactivate” enzyme &
prevent a reaction form occurring (therefore, changing phenotype)
Expression of Genes
• Examples:– tobacco: green dominant & albino recessive
• however color is also affected by environment– If no sunlight, green color cannot be expressed due to
lack of chlorophyll production» put in light green will appear b/c chlorophyll
being produced
Expression of Genes
Unit 9: Human Genetic Disorders
• What causes genetic disorders?– DNA mutation (usually recessive) or chromosome
abnormalities (in # or structure) that cause the production of abnormal proteins
• How can we group genetic disorders?1. autosomal recessive disorders
(*most genetic disorders)• allele is recessive & found on a chromosome from
pairs 1 – 22 (autosomes or body chromosomes)– cystic fibrosis (CF), sickle-cell anemia, Tay-Sachs disease
2. autosomal dominant disorders• allele is dominant & found on a chromosome from
pairs 1 – 22 (autosomes or body chromosomes)– Huntington’s Disease
Human Genetic Disorders
3. sex-linked disorders• allele (which is usually recessive) is found on the 23rd
pair of chromosomes (sex chromosomes)… Usually on the X chromosome
– hemophilia, color blindness
4. chromosomal abnormality disorders• result from errors in chromosome # or structure
– Down Syndrome (trisomy 21), Klinefelter’s Syndrome (XXY)
Human Genetic Disorders
Autosomal Recessive Disorders• What genotype(s) must a
person have to be affected?– homozygous recessive (gg)
• cystic fibrosis• sickle-cell anemia• Tay-Sachs Disease
• Can someone be a carrier? Why/why not?– yes
• if heterozygous (Gg), person carries gene, but isn’t affected
– due to having the “normal” dominant gene
Autosomal Dominant Disorders• What genotype(s) must a person
have to be affected?– homozygous (GG) or heterozygous
(Gg) b/c allele is dominant• Huntington’s Disease
• Can someone be a carrier? Why/why not?– No
• even if person is heterozygous (Gg), will have disorder
– due to dominant “disease” gene blocking “normal” recessive gene
Sex-linked Disorders• Recall… hemophilia is X-linked & recessive
– What are the possible genotypes & phenotypes? Can someone be a carrier?• XHXH = normal female• XHXh = carrier female (but not affected) • XhXh = female w/ hemophilia• XHY = normal male• XhY = male w/ hemophilia
Sex-linked Disorders
–Why can’t a male be a carrier?• b/c only has one X chromosome w/ allele…
so either has it or doesn’t–Ex. mom = carrier & dad = normal:
• Make a Punnett square.–genotypic ratio?
» 1 XHXH: 1 XHXh: 0 XhXh: 1 XHY: 1 XhY
–phenotypic ratio?» 1 normal female: 1 carrier female (not affected) :
0 female w/ hemophilia: 1 normal male: 1 (affected) hemophiliac male
• Recall… colorblindness is X-linked recessive– What are the possible genotypes & phenotypes?
Can someone be a carrier?• XCXC = normal female• XCXc = carrier female (but not affected) • XcXc = colorblind female• XCY = normal male• XcY = colorblind male
Sex-linked Disorders
Ishiharatest forred-greencolor-blindness
–In this Punnett square, what are genotypes & phenotypes of parents?• father:
–genotype = XCY–phenotype = normal
• mother:–genotype = XCXc
–phenotype = carrier (but she is not affected)
Sex-linked Disorders
Chromosomal Abnormalities in Number
• What causes an abnormal number of chromosomes?– non-disjunction
• failure of paired chromosomes to separate during meiosis 1 or meiosis 2
Disorders Due to Abnormal Chromosome #• What is Down Syndrome (trisomy 21)?
– when person has 3 copies of chromosome # 21• What causes Down Syndrome (trisomy 21)?
– non-disjunction• failure of paired chromosomes to separate during
meiosis 1 or meiosis 2
Disorders Due to Abnormal Chromosome #• What is Klinefelter’s Syndrome?
– a sex-chromosome disorder in which males have extra copy of X chromosome• XXY (or 47, XXY b/c 47 total chromosomes)
• What causes Klinefelter’s Syndrome?– non-disjunction
• failure of paired chromosomes to separate during meiosis 1 or meiosis 2
Chromosomal Abnormalities in Structure
• What is causes structural abnormalities in chromosomes?− pieces are
added, deleted, inverted, or translocated
Detecting Abnormalities• Karyotyping
–“picture of human chromosomes”• From blood sample
–Can detect extra chromosomes or chromosomal abnormalities (additions, deletions, inversions, translocations)
• Amniocentesis– sample of fluid surrounding fetus
(karyotype then made)• Can detect Down Syndrome
– 14th + week of pregnancy
• Chorionic villus biopsy– sample of cells from chorion (part of structure by
which fetus linked to mother)– 9th + week of pregnancy
Detecting Abnormalities
Pedigree Charts• A family tree (chart) of genetic history of
family over several generations
Square = male
Circle = female
Shaded = studied trait
Marriage = horizontal line
Offspring = vertical line
Review & Animations
• Vocab interactive– http://nortonbooks.com/college/biology/animations/ch10a02.htm
• Crosses– http://www.sonefe.org/online-biyoloji-dersleri/grade-12/monohybrid-cross/
• Drag & drop genetics– http://www.zerobio.com/drag_gr11/mono.htm
• Various– http://www.abpischools.org.uk/page/modules/genome/dna4.cfm?coSiteNavig
ation_allTopic=1
• Genetic disorders– http://www.humanillnesses.com/original/Gas-Hep/Genetic-Diseases.html