Complex Inheritance &

Human Heredity

Chapter 11

Chapter 11.1Basic Patterns of Human Inheritance

A recessive trait is expressed when an individual holds TWO recessive alleles (homozygous recessive)

Example s recessive genetic disorders include:Cystic fibrosisAlbinismTay-Sachs diseaseGalactosemia

Individuals heterozygous for a recessive disorder will express the dominant trait (no disorder) but will be a carrier of the disorder

Recessive Genetic Disorders

Some disorders are caused by dominant genes

Those people without the disorder are homozygous recessive

Examples of Dominant Genetic Disorders:Huntington’s DiseaseAchondroplasia

Huntington’s Disease - http://www.youtube.com/watch?v=65xf1olEpQM

Dominant Genetic Disorders

A diagram that traces the inheritance of a particular trait through several generations

Read about Pedigrees on pp. 299-301Complete the Mini-Lab on p. 300

Pedigrees

Chapter 11.2Sex Determination, Sex-Linked Traits, &

Sex Influenced Traits

Complex Inheritance Patterns

RED Flower x WHITE Flower ---> PINK Flower

INCOMPLETE DOMINANCE

 With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

There are foxes that roam the forests in northern Baltimore County. They come in three colors, blue, gold, and green. This trait is controlled by a single gene with incomplete dominance.

A homozygous (BB) individual is blue, a homozygous (bb) individual is gold, and a heterozygous (Bb) individual is green.

What would be the genotypes and phenotypes of the offspring if a blue fox were crossed with a gold one?

Problem:

red x white ---> red & white spotted

CODOMINANCE

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. 

Longhorns may be white (CWCW), red (CRCR) or roan (CRCW). Roan longhorns have a mixture of both white hairs and red hairs due to a codominant gene.

"A single copy can be expressed by just a few white hairs on the face or extremities, unevenly roaned patches, or an even mix of white and colored hairs all over the body. Two copies produce an almost white animal, with some pigment around the ears.”

What would the phenotypes and genotypes of the offspring if a red male were crossed with a white female?

Sickle Cell Disease Sickle cell disease is caused by the

allele that controls the formation of the protein hemoglobin.

Hemoglobin is the part of the red blood cell that carries oxygen.

The allele for normal hemoglobin (A) results in red blood cells that are disc-shaped.

The sickle cell allele (S) changes the hemoglobin and results in red blood cells that are sickle -shaped.

People who are heterozygous for these two alleles (AS) have both normal and sickle-shaped red blood cells.

PROBLEM:What is the risk that two people heterozygous for the sickle-shaped allele will have a child with sickle cell disease?

A single gene that has more than two possible alleles.

MULTIPLE ALLELES

ALLELE IA IB i

CODES FOR Type "A" Blood Type "B" Blood Type "O" Blood

Notice that that the allele for "O" (i) is recessive to the alleles for "A" & "B". With three alleles we have a higher number of possible combinations in creating a genotype.

GENOTYPES IAIA IAi

RESULTING PHENOTYPES Type A Type A

IBIB IBi

Type B Type B

IAIB Type AB

ii Type O

A woman with Type O blood (ii) and a man who is Type AB (IAIB) have are expecting a child. 

What are the possible blood types of the child?

Problem:

One allele hides the effects of another allele

Epistasis

Primula flowers produce a blue pigment with the dominant Gene K

However, the presence of a dominant Gene D will mask effect of Gene K

(KKDD, KKDd, KkDD, KkDd will all appear another color)

Gender is determined by a pair of chromosomes called the sex chromosomes.two types: X and Y

females have two X chromosomes

males have one X and one Y chromosome

In humans, the remaining 22 pairs of chromosomes are homologous and are referred to as autosomes.

Sex Determination

Dosage CompensationX chromosome is larger than

Y ‘X’ contains 900-1400 genes ‘Y’ contains fewer than 100

genes – mostly related to “maleness”

In females, one of the X chromosomes is inactivated. X-inactivation occurs randomly in

each body cell. Form of dosage compensation. Inactivated X-chromosome

condenses into a darkly staining region called a Barr body.

Example: Calico color in cats

Calico cat

Sex-Linked TraitsTraits controlled by genes on the sex

chromosomes are called sex-linked traits. Mostly X-linked traits; very few Y-linked traits. Males disproportionately affected by recessive

alleles on the X chromosome. Examples:

Red-green color blindness Hemophilia

Red-Green Colorblindness A recessive X-linked trait Affects 8-12% of males in the United States; less than 0.5%

of females. Results when the color-detecting cones in the retina of the

eyes function ‘poorly’ in discriminating between red and green colors.

Normal vision

Red-green colorblind vision

Color blindness Problem:PROBLEM: A man with normal vision and

a woman who is heterozygous for the colorblind allele want to have a child. ‘B’ is the allele for normal

vision ‘b’ is the allele for color

blindness

What is the probability that their child will be colorblind?

Could they have a colorblind daughter?

XB YXB XBXB XBYXb XBXb XbY

SOLUTION: 25% No, they cannot have a colorblind daughter. The only child that

could be affected would be a boy.

Red-Green Color BlindnessOther

Colorblindness Tests

Variations in Red-Green Color Blindness

HemophiliaA recessive X-linked

disorderResults in a delayed

clotting of bloodWill it be more prevalent

in males or females? Why?

A man with hemophilia has children with a woman who is a carrier for hemophilia. What is the chance that their next child will have hemophilia? Their next son? Their next daughter?

Sex-Linked Practice Problem:

Sex Influenced TraitsTraits located on the autosomes can sometimes

be affected by the proportion of sex hormones produced by the body. Ultimately sex hormone production is prescribed by

the sex chromosomes. These are called sex-influenced traits Example:

Male pattern baldness – gene expressed in the presence of testosterone

Polygenic TraitsTraits affected by

more than one geneExamples:

Skin colorEye colorHeight

ADAM Inc., Male Pattern Baldness. 14 Apr 2008. U.S. National Library of Medicine 16 Apr 2008 <http://www.nlm.nih.gov/medlineplus/ency/imagepages/17083.htm>

ADAM Inc., Various Tests for Color Blindness. 2007. New York Times Company. 16 Apr 2008 <http://www.nytimes.com/imagepages/2007/08/01/health/adam/9962Colorblindnesstests.html>.

Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.

Color in Computer Graphics. 25 Feb 1998. Cornell University Program in Computer Graphics. 16 Apr 2008 <www.graphics.cornell.edu/online/tutorial/color/>.

Waggoner, Terrace L.. "About Color Blindness (Color Vision Deficiency)." Colors for the Color Blind. U.S. Naval Hospital, Pensacola, FL. 16 Apr 2008 <http://www.toledo-bend.com/colorblind/aboutCB.html>.

"X chromosome." Genetics Home Reference: Your Guide to Understanding Genetic Conditions. 14 Apr 2008. U.S. National Library of Medicine. 16 Apr 2008 <http://ghr.nlm.nih.gov/chromosome=X>.

“X chromosome.” Photo Researchers, Inc. 16 Apr 2008.

References

Chapter 11.3Karyotypes and Nondisjunction

Chromosomes and Human Heredity

Some inherited traits can be identified at the chromosome level.

Geneticists use karyotypes.Chromosomes are stained.A photomicrograph is taken of a cell’s

chromosomes during metaphase.Chromosomes pairs are arranged in order of

decreasing size.

Karyotypes

Example Karyotype

Fetal Tests

Test Benefit Risk

Amniocentesis• Diagnosis of chromosome abnormalities• Diagnosis of other defects

• Discomfort for expectant mother

• Slight risk of infection• Risk of miscarriage

Chorionic villus sampling

• Diagnosis of chromosome abnormality• Diagnosis of certain genetic defects

•Risk of miscarriage•Risk of infection•Risk of newborn limb defects

Fetal blood sampling

• Diagnosis of genetic or chromosome abnormality

• Checks for fetal blood problems and oxygen levels

• Medications can be given to the fetus before birth

• Risk of bleeding from sample site

• Risk of infection• Amniotic fluid might leak• Risk of fetal death

Amniocentesis procedure

NondisjunctionCell division in which either the homologous

pairs or sister chromatids do not separate correctly, resulting in gametes with an abnormal number of chromosomes.

Types of NondisjunctionMonosomy – having only one of a particular type

of chromosome.Trisomy – having a set of three chromosomes of

one kinde.g. Down syndrome (Trisomy 21)

Results in distinctive facial features, short stature, heart defects, and mental disability

Nondisjunction in Sex Chromosomes Females can survive with only one X chromosome or with

three. Males can survive with either an extra X or an extra Y. Males cannot survive with only a Y chromosome.

Genotype

XO XXX XXY XYY OY

Phenotype

Female with Turner’s

syndrome (sterile)

Nearly normal female

Male with Klinefelter’s syndrome (sterile)

Normal or nearly normal

male

Results in death

Biggs, Haggins, Holliday. Biology (Glencoe Science)(Teacher Wraparound Edition). New York: GLENCOE MCGRAW HILL, 2007.

Farabee, M. J. Human Genetics. 2001. 01 Apr. 2009 <http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookhumgen.html>.

Downschild. Digital image. About Downs Syndrome. 2008. The Coventry and Rugby Down's Syndrome Support Group. 01 Apr. 2009 <http://www.downssupport.org.uk/aboutdowns.html>.

"Genetics and pregnancy loss." Miscarriage Management™ -. 2009. Sydney IVF Limited. 01 Apr. 2009 <http://www.miscarriage.com.au/basepage.cfm?id=16>.

Karyotype. Digital image. AP Biology. 01 Apr. 2009 <http://www.nkellogg.com/apbiology.htm>.

Nondisjunction. Digital image. 01 Apr. 2009 <http://porpax.bio.miami.edu/~cmallery/150/mendel/heredity.htm>.

References