Complex Patterns of Inheritance and Meiosis inheritance mistakes

More Complicated Situations…

1. Incomplete dominance

Multiple Alleles

IAIA = A; IAi = A IBIB=B; IBi = B

IAIB= ABii = O A, B – codominant

i - recessive

Other situations:• Pleiotropy – one gene affects several

phenotypes• Epistasis – one gene affects the phenotypic

expression of another gene• Polygenetic inheritance – additive effect of

two or more genes on a single phenotypic character

Human Disorders that follow Mendelian Patterns of Inhertitance

• Recessively inherited disorders: albinism, cystic fibrosis, Tay-Sachs disease, sickle-cell anemia

• Dominantly inherited disorders: Achondroplasia, Huntington’s disease

Cystic Fibrosis

is an autosomal recessive genetic disorder that affects mostly the lungs but also the pancreas, liver, and intestine. Difficulty breathing is the most serious symptom and results from frequent lung infections.

Tay-Sachs Disease

is a rare autosomal recessive genetic disorder. In its most common variant), it causes a progressive deterioration of nerve cells and of mental and physical abilities that begins around six months of age and usually results in death by the age of four.

Achondroplasia

Huntington DiseaseLate Acting – middle age

Western Europeans

Huntington's disease (HD) is a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and behavioral[1] symptoms. The disease is caused by an autosomal dominant mutation in either of an individual's two copies of a gene called Huntingtin, which means any child of an affected person typically has a 50% chance of inheriting the disease.

Screening for inherited disorders

Pedigrees

Sex-linked traitsLocated on Sex chromosomes: X or Y

Females XX; Males XY

X-linked recessive

Hemophilia, color blindness

Human Disorders Due to Chromosomal Alterations

• Alterations of chromosome number and structure– Are associated with a number of serious human

disorders

Abnormal Chromosome Number• When nondisjunction occurs

– Pairs of homologous chromosomes do not separate normally during meiosis

– Gametes contain two copies or no copies of a particular chromosome

Figure 15.12a, b

Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

Gametes

n + 1n + 1 n 1 n – 1 n + 1 n –1 n nNumber of chromosomes

Nondisjunction of homologouschromosomes in meiosis I

Nondisjunction of sisterchromatids in meiosis II

(a) (b)

Aneuploidy of Sex Chromosomes• Nondisjunction of sex chromosomes

– Produces a variety of aneuploid conditions

Down Syndrome

Down syndrome– Is usually the result of an extra chromosome 21, trisomy 21

Figure 15.15

• Klinefelter syndrome– Is the result of an extra chromosome in a male,

producing XXY individuals• Turner syndrome

– Is the result of monosomy X, producing an X0 karyotype