Human GeneticsChapter 12

Human Genetic Traits

• Do you think this trait is dominant or recessive?

Widow’s Peak?

DOMINANT

PTC Tasting

• Can you taste the PTC????

•DOMINANT

Earlobe Shape?• Is your earlobe attached?

• Do you think this trait is dominant or recessive? RECESSIVE

Dimples?

DOMINANT

Human Inheritance

• Humans - 23 pairs of chromosomes– These are made of about 100,000 genes

• Scientists study disease causing genes because they can easily be traced– Pedigree – a family record that shows how a

trait is inherited over several generations.

PedigreeA Pedigree of Hemophilia in the Royal Families of Europe

Pedigreepage 299

• Carriers – usually, Heterozygous

-do not express the recessive allele, but pass it to their offspring

Square = Male

Circle = Female

No shading = normal

Shaded = displays trait

Half/Half = Carrier

Pedigree Practice

• Construct a family pedigree of two unaffected parents with a child who suffers from cystic fibrosis.

Pedigree Practice

• Suppose both parents can roll their tongues but their son cannot. Draw a pedigree showing this trait, and label each symbol with the appropriate genotype.

Pedigree Practice

• Describe the pedigree of a boy who has galactosemia. His father has galactosemia, his paternal grandparents are phenotypically normal. His mother and maternal grandparents are both phenotypically normal.

• A boy is an albino. His mother is also an albino. His father is phenotypically normal. However, his paternal grandfather is an albino. The other 3 of his grandparents are phenotypically normal.

• A brother and a sister both are hemophilic. Neither parent shows this trait, however the maternal grandmother suffers from this disorder. The other grandparents are phenotypically normal.

On Your Own

• Page 300

• Mini Lab 11.1 – Investigate Human Pedigrees

Simple Dominant Heredity

• Many traits are inherited just as the rule of dominance predicts.

• Remember, in Mendelian inheritance, a single dominant allele inherited from 1 parent is all that is needed for a person to show the dominant trait.

Patterns of Inheritance• Traits controlled by a Single Allele

– >200 traits are determined by a single dominant allele• Ex. Huntington’s Disease

– >250 other traits are determined by homozygous recessive alleles

• Ex. Cystic Fibrosis, PKU, blue

people in Eastern Kentucky

Sometimes Heredity Follows Different Rules

1. Incomplete Dominance: Appearance of a third phenotype

2. Sex-linked inheritance

3. Codominance: Expression of

both alleles

1. Incomplete Dominance• Incomplete dominance - Cross

between organisms with 2 different phenotypes

-produces offspring with a 3rd phenotype that is a blending of the parental traits.  – RED Flower x WHITE Flower ---

> PINK Flower

• R = allele for red flowers W = allele for white flowers red x white ---> pink RR x WW ---> 100% RW

2. Sex determination• In humans, the diploid number of

chromosomes = 46 (23 pairs)– There are 22 matching pairs of homologous

chromosomes called autosomes.– The 23rd pair differs in males and females, they

determine the sex of an individual (sex chromosomes) • X females (XX)• Y males (XY)

*Complete a punnett square to determine the expected ratio of males to females produced given their possible gamete contribution

Sex-linked inheritance• Traits controlled by genes located on sex

chromosomes are called sex-linked traits

• Sex-Linked Traits are found only on the X chromosome– Ex. Hemophilia (recessive)– Ex. Patterned Baldness

• Homozygous baldness-both will lose hair

• Heterozygous-men will lose hair but women will not

– Ex. Colorblindness (recessive)

Color Blindness Activty • http://colorvisiontesting.com/ishihara.htm • http://colorvisiontesting.com/online%20test.htm • Draw your family pedigree for color blindness!

• Predict what your children genotype could be if: – Boys – you married a woman who carried the trait for being color

blind on her X chromosome– Girls – you married a man who was color blind

Patterns of Inheritance

• Complete a punnett square to show how the allele for red eye color is a sex-linked trait

3. Codominance• Codominance - the "recessive" & "dominant"

traits appear together in the phenotype of hybrid organisms.

• Example:

red x white ---> red & white spotted

• R = allele for red flowers W = allele for white flowers red x white ---> red & white spotted RR x WW ---> 100% RW

Examples of Codominance1. Roan fur in cattle

– Cattle can be:1. red (RR = all red hairs)2. white (WW = all white hairs)3. roan (RW = red & white hairs together)

2. Human blood type: AB-2 types of protein ("A" & "B") appear together on the surface of blood cells

– How to determine Blood Types:– 4 possible blood types (in order from most

common to most rare): O, A, B and AB.

– O blood type = individual who is homozygous recessive (ii) and does not have an allele for A or B.

• Blood types A and B are codominant alleles. – Recessive allele i (for blood type O) is only

expressed when 2 recessive alleles are present.

– Individuals who have blood type A:• Genotype = IAIA or IAi

– Individuals who have blood type B:• Genotype = IBIB or Ibi

– Individual who has blood type AB:• Genotype = IAIB

– Individual who has blood type O:• Genotype = ii

Human Blood Types

– Blood transfusion can only take place between 2 people who have compatible types of blood.

– Human blood is separated into different classifications because of the varying proteins on the surface of blood cells.

– These proteins are there to identify whether or not the blood in the individual's body is it's own and not something the immunity system should destroy.

**IMPORTANCE in Real-Life**

Blood type practiceUse a Punnett Square!

1. A woman has type A blood. Her father has type O blood. The woman marries a man with type O blood. What is the chance that they will have a child with type A blood?

2. What is the chance that the couple from question 1 will have a child with type AB blood?

*Show me your answers when you are finished. Keep these in your notes!

Sickle Cell Anemia• Read about

Codominance on pg. 302-303

• Define Codominance.

• Explain Sickle-cell Disease in ½ page.

• Complete Data Analysis Lab 11.1 on the bottom ½ of the page.

Sickle Cell Anemia Cont.

• A Mutation Story:

http://www.teachersdomain.org/resource/tdc02.sci.life.gen.lp_disorder/

Nondisjunction

• Nondisjunction is the failure of homologous chromosome pairs to separate properly during meiosis. The result of this error is a cell with an abnormal (too few or too many) number of chromosomes.

Nondisjunction

Activity: Human Karyotype

Patterns of Inheritance

Child with Trisomy 21

Disorders due to Nondisjunction Cont. – Klinefelter’s (XXY)-male w/ some female

traits

– Turner’s (XO)-female appearance• Single Y chromosome do not survive

• Typically Sterile

Environmental Effects

• Genes are inherited from parents, but sometimes their expression is modified by environmental factors.

• An example is the snowshoe hare we discussed earlier in the year-these hares have dark fur in the summer and white fur in the winter.

Snowshoe Hare

Detecting Human Genetic Disorders

• Genetic Screening – examination of genetic makeup– Karyotype: a picture of chromosomes

grouped in pairs and arranged in sequence.

– Screening of Blood: look for certain proteins• Genetic Counseling-medical guidance

informing of problems that could affect their offspring.

Detecting HumanGenetic Disorders

– Amniocentesis: removal of small amount of amnionic fluid surrounding the fetus

– Chorionic Villi Sampling: tissue that grows between the mother’s uterus and the placenta (between the 8th and 10th week)

– Screening Immediately after Birth:• Ex PKU (Phenylketonuria)-body cannot

metabolize the amino acid phenylalanine– Special diet lacking phenylalanine

Karyotyping

• http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping2.html