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

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?

• 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– crossing over during

prophase 1– “independent assortment”

of chromosomes based on alignment during metaphase 1

Genes

Genes & Alleles• What is a gene?

– section of chromosome that 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

• prevents expression of (“masks”/“hides”) recessive trait

– recessive• seen only when pure (homozygous) for trait

• Represented with letters– usually first letter of dominant trait

• same letter used for dominant & recessive– CAPITAL = dominant– lowercase = recessive

Allele Combinations• If both alleles are:

– the same• homozygous (pure) dominant (ex. AA)• homozygous (pure) recessive (ex. aa)

– different• heterozygous (hybrid) (ex. Aa)

Genotype vs. Phenotype• genotype = actual genetic

make-up of individual (alleles)– codes for phenotype (trait)

– represented by 2 letters• represent alleles from mom & dad

– ex. PP, Pp, pp

• phenotype = outward (physical) expression of the genotype– trait we “see”

• (due to) the protein that is produced

– usually represented by an adjective

• ex. purple, white, etc.

Genotype is Expressed as a Phenotype• Ex. Let P = purple & p = white

– homozygous (pure) dominant• genotype PP• phenotype = purple

– heterozygous (hybrid)• genotype Pp• phenotype = purple

– dominant trait “masks/hides” recessive trait

– homozygous (pure) recessive• genotype pp• phenotype = white

• Punnett Squares– Help predict the results of crosses (mating)

• Letters along top & side represent possible alleles in gametes of each parent

• Boxes represent possible allele combinations (genotypes & resulting phenotypes) in offspring

– Can be used to determine probability and ratios

Predicting Traits in Offspring

Making a Punnett Square• Parents are Tt & tt genotypes…

– So… Tt x tt is our cross

Passing Traits to Offspring & Probability

• Probability– the chance an

event will occur– What is the chance

of getting heads? Tails?

• If you flip two coins, of getting 2 heads? 2 tails?

• What is the chance of a couple having a boy? A girl? Of having four boys? Five girls?

Passing Traits to Offspring & Ratios

• genotypic ratio = probable ratio of genotypes in offspring of a cross– Ex. If cross Pp & Pp

• 1PP : 2Pp : 1 pp

• phenotypic ratio = probable ratio of phenotypes resulting from the genotypic ratio

• Ex. If cross Pp & Pp• 3 purple : 1 white

Passing Traits to Offspring & Ratios• expected ratio =

ratio expected based on probability (Punnett Square)

• observed ratio = what actually occurs

• Why would these be different?

• If one parent is homozygous dominant & other is homozygous recessive– each parent can only produce

gametes with 1 type of allele

– All offspring will always have:• heterozygous (hybrid) genotype

– ex. Ss or Pp

• dominant phenotype– ex. smooth or purple

Passing Traits to Offspring

• If both parents are heterozygous– each parent can produce

gametes with 2 types of alleles

– Offspring will always have:• 1 homozygous dominant :

2 heterozygous : 1 homozygous recessive genotype ratio

– ex. 1 SS : 2 Ss : 1 ss

• 3 dominant phenotype : 1 recessive phenotype ratio

– ex. 3 smooth : 1 wrinkled

Passing Traits to Offspring

Phenotypegenotype

Phenotypegenotype

Pp

Pp

Pp

Pp

P p

p P

Gregor Mendel – the Father of Genetics

1822-1884

Mendel’s Experiments• Studied garden pea plants

– 7 different traits with clearly different forms• Tried to determine how these traits were passed f

rom parent to offspring

Mendel’s Experiments• Mated pure

purple parent (PP) & pure white parent (pp)• All offspring had:

• purple phenotype

• heterozygous (hybrid) genotype

–Pp

• Heterozygous (hybrid) offspring allowed to self- pollinate– So… Pp x Pp

• New offspring weren’t all purple

Mendel’s Experiments

Mendel’s Principle of Dominance• Mendel noted that one

form dominates over the other–dominant trait prevents

the expression of the recessive trait• Ex. In peas, purple x

white gives all purple offspring

–PUPRLE = dominant

–white = recessive

Dominant/Recessive is Not Always the Mode 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• humans have 46

chromosomes (in body cells)– 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?

What is the probability of having a son?

A daughter?

Sex-linked Inheritance• X & Y chromosomes not fully homologous

– X is bigger & carries more genes

• Males will have only 1 allele for traits carried only on X– called X-linked or sex-linked

• Ex.: – In Drosophila (fruit flies) eye color

– In humans hemophilia & colorblindness

– X-linked traits & disorders are more common in males• Why???

• Predictions made using Punnett square– Consider the sex chromosomes (X or Y) & genes

they carry (shown as superscript/exponent) together as a unit…

• ex. XG (= dominant gene), Xg (= recessive gene), Y (= no gene)

– If a female is heterozygous, she does not show the trait/have the disorder, but is a carrier

• can pass gene to offspring

Sex-linked Inheritance

XG female Xg

XG XG XG Xg

XG Y Xg Y

• Ex. In Drosophila (fruit flies) eye color– What are the sex,

genotype, & phenotype of each offspring?• Are there any female

carriers for the white eye gene?

Sex-linked Inheritance

Codominance• heterozygote (hybrid) shows both traits

– shown by 2 different capital letters

• Ex. Roan cow

– phenotype = mix of both red & white hairs

– genotype = RW

Multiple Alleles• more than 2 different forms

of an allele exist– but individual still has just 2

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

• IA (A)• IB (B)• i (o)

– also exhibits codominance• IA = IB (A & B are codominant)

– i (o is 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?

Human Genetic Disorders

• Due to DNA mutation (usually recessive) or chromosome abnormalities (in # or structure)– Causes production of abnormal proteins

• Examples:– Autosomal recessive disorders (***most genetic disorders)

» Cystic Fibrosis

» Sickle-cell Anemia

» Tay-Sachs Disease

– Autosomal dominant disorders» Huntington’s Disease

– Sex-linked disorders» Hemophilia

» Color Blindness

– Chromosomal abnormality disorders» Down Syndrome (trisomy 21)

» Klinefelter’s Syndrome (XXY)

Human Genetic Disorders

Autosomal Recessive Disorders• To be affected, must

be homozygous b/c allele is recessive– Cystic Fibrosis

– Sickle-cell Anemia

– Tay-Sachs Disease

Autosomal Dominant Disorders• To be affected, can be

homozygous or heterozygous b/c allele is dominant– Huntington’s Disease

Sex-linked Disorders•Hemophilia•Color blindness

Sex-linked Disorders

– If mother is carrier & father has hemophilia:

• genotypic ratio?• phenotypic ratio?

– If mother is carrier & father is normal:

• Make a Punnett square– genotypic ratio?– phenotypic ratio?

•Hemophilia is X-linked recessive

• Colorblindness is X-linked recessive– In this Punnett square, what are

the genotypes & phenotypes of the parents?

Sex-linked Disorders

Ishiharatest forred-greencolorblindness

Chromosomal Abnormalities in Number

• abnormal number of chromosomes:– Caused by non-

disjunction• failure of paired

chromosomes to separate during meiosis 1 or meiosis 2

Chromosomal Abnormality Disorders• Down Syndrome

(trisomy 21)– person has 3 copies

of chromosome # 21• Caused by non-

disjunction

• Klinefelter’s Syndrome– Sex chromosome

disorder• Males have extra copy of

X chromosome– XXY (or 47, XXY b/c 47

total chromosomes)

• caused by non-disjunction

Chromosomal Abnormality Disorders

Chromosomal Abnormalities in Structure

• abnormal structure of chromosomes:− added, deleted,

inverted, or translocated pieces

Detecting Abnormalities

• karyotyping– “picture of human

chromosomes”• From blood

sample– Can detect extra

chromosomes or chromosomal abnormalities

• Amniocentesis– sample of fluid

surrounding fetus

• can detect Down Syndrome

• Chorionic villus biopsy– sample of

cells from chorion

Detecting Abnormalities

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?

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