Unit 8: Genetics & HeredityUnit 9: Human Genetic Disorders

Ch. 26: Inheritance of Traits& Ch. 27: Human Genetics

• 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… don’t know

which sperm will fertilize which egg…

• get ½ of 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• different forms of genes for the

same trait are called “alleles”

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

• What are Punnett Squares?– a way to predict the results of crosses (mating)

• letters outside represent possible alleles in gametes of each parent

– top = one parent & side = other parent

• letters inside boxes represent possible allele combinations (genotypes) in offspring (& phenotypes)

– can be used to determine probability and ratios

Predicting Traits in Offspring

BB Bb

Making a Punnett Square• 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 (Punnett Square)

• What is an “observed ratio”?– ratio we actually get

• Why would these be different?– fertilization is random– some embryos die

during early stages

Gregor Mendel• Father of Genetics

– 1822-1884

• studied garden pea plants– 7 different traits with

clearly different forms• tried to determine how t

hey were passed from parent to offspring

Mendel’s Experiments• What happened when Mendel mated

a pure purple parent (PP) & a pure white parent (pp)?–all offspring had:

• purple phenotype

• heterozygous (hybrid) genotype

–Pp

• What happened when Mendel let the heterozygous (hybrid) offspring from his first experiment self-pollinate?– So… Pp x Pp

• new offspring weren’t all purple…

Mendel’s Experiments

Mendel’s Principle of Dominance• What did Mendel notice

from his experiments?– that one form dominates

over the other• …dominant trait prevents

the expression of the recessive trait

– What trait was dominant in these plants?

» PUPRLE = dominant– What trait was recessive?

» white = recessive

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• sex determination• sex-linked traits• codominance• multiple alleles

Sex Determination• How many chromosomes do

humans have (in body cells)?– 46… 23 pairs

• pairs 1 – 22 = autosomes (body chromosomes)

• 23rd pair determines gender = sex chromosomes

– XX = female

– XY = male

• Which parent’s chromosomes determines if the offspring will be a boy or girl???? Why?– Dad’s b/c he is the only one that

can give a Y; mom always gives X.

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?– 1 b/c only have 1 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

– X-linked 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)

• How do we make predictions made using Punnett squares for sex-linked traits?– Consider the 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)

• 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 XG XG Xg

XG Y Xg Y

• Drosophila (fruit fly) eye color is sex-linked– What are the sex, genotype, & phenotype of each

offspring? Are there any carriers for the white eye gene?

• Left picture: 2 females with red eyes = XRXr (carrier white eye gene) & 2 males with white eyes = XrY

• Right picture: female w/ red eyes = XRXR, female w/ red eyes = XRXr (carrier white eye gene), male w/ red eyes = XRY, & male w/ white eyes = XrY

Sex-linked Inheritance

XR Y

XR XR XR Y

XR Xr Xr Y

Multiple Alleles & Codominance• What is meant by

multiple alleles?– more than 2 different

forms of an allele exist• but individual still has

just 2

– Ex. human blood types• (3) multiple alleles

– A (IA)– B (IB)– o (i)

•How many possible genotypes are there?

•How many phenotypes?

•Can you spot the blood type that is the result of codominance?

Multiple Alleles & Codominance• What is meant by

codominance?– both alleles are

“expressed” equally

– Ex. human blood types also exhibit codominance (as well as multiple alleles)• A & B are codominant and

are “expressed” equally– A = B (codominant)– o (recessive)

» So… (A = B) > o

•How many possible genotypes are there?

•How many phenotypes?

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, 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

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

• b/c if heterozygous (Gg), person carries the 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?– can be homozygous (GG) or

heterozygous (Gg) b/c allele is dominant

• Huntington’s Disease

• Can someone be a carrier? Why/why not?– No

• b/c even if person is heterozygous (Gg), person will have disorder

– due to dominant “disease” gene blocking “normal” recessive gene

Sex-linked Disorders• Remember from earlier… 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

– 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?

– phenotypic ratio?

1 XHXH: 1 XHXh: 0 XhXh: 1 XHY: 1 XhY

1 normal female: 1 carrier female : 0 female w/ hemophilia: 1 normal male: 1 hemophiliac male

• Remember from earlier… 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

– In this Punnett square, what are the genotypes & phenotypes of the parents?

• father:– genotype = XCY & phenotype = normal

• mother:– genotype = XCXc & phenotype = carrier

Sex-linked Disorders

Ishiharatest forred-greencolor-blindness

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 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 Down Syndrome (trisomy 21) & 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

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?

coSiteNavigation_allTopic=1

• Genetic disorders– http://www.humanillnesses.com/original/Gas-Hep/Genetic-Diseases.html