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

PP

Pp

pp

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

Parent

First filial

Second Filial

Crossed two F1 plants to get F2

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

Dihybrid Cross

2 traits with 2 forms

• 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

When both parents are hybrid for both traits,Always get 9 : 3 : 3 : 1 ratio

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?

antigen

antigen

antigens

No antigens

• 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:–Himalayan hare: temperature & fur color…

warm = white, cold = black

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