Chapter 12: Patterns of Heredity & Human Genetics Section 1: Mendelian Inheritance of Human Traits
Making a Pedigree When genetic inheritance is represented by a picture, this is called a pedigree. Pedigrees are used by geneticists to map inheritance from generation to generation.
It is a diagram made of symbols that identify three things: 1. Male or female 2. Individuals affected by the trait being studied 3. Family relationships
S ymbols of a pedigree: ***need to know*** Carrier heterozygous individual
Constructing and Reading a pedigree ***an inverted v means the married couple had twins*** (know) ****a horizontal line between a male and female with a strike means the persons are divorced.**** (Need to know )
Constructing and Reading a pedigree ***Roman Numerals (I, II, III) refers to the generations.*** *Arabic numbers refers to individuals. (1, 2, 3, 4, 5, ) 321456 1 1 2 2 345 I. III. II.
Reading the pedigree How many generations are there? How many children did II-1 have? II-7? How are III-5 and III-2 related? Who is III-2 in reference to I-2 ? 32145 61 1 2 2 345 I. III. II. 7
Types of Pedigrees Pedigrees can be: a.) autosomal *There is a 50/50 ratio between men and women of affected individuals. b.) X- linked *Most of the males in the pedigree are affected.
*X-linked are carried by females, but not expressed in females. *X-linked are expressed most often in MALES. *In males, to express an X-linked disorder, he only needs to have one gene. (XY - heterozygous) *In females, to express an X-linked disorder, she needs TWO alleles to show the disorder. (XX homozygous recessive) Ex: Colorblindness, hemophilia, baldness Facts about X-linked Disorders *carried on the X-chromosome Colorblindness Pedigree
Colorblindness Tests Test Name: Ishihara Test Colorblind sees: yellow square Normal color: yellow square & faint brown circle Colorblind sees: the number 17 Normal Color sees: the number 15
Simple Recessive Heredity Most genetic disorders are caused by recessive alleles. This means the disorder is inherited when both parents have a recessive allele.
Common Recessive Disorders Cystic Fibrosis (CF): A defective protein in the plasma membrane of cells causes thick mucus to build up in the lungs and digestive system. Mostly found among white Americans.
Pedigree for Cystic Fibrosis
Tay-Sachs Disease: The absence of an enzyme causes lipids to accumulate in the tissues and nerve cells of the brain. blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in and other neurological symptoms Death usually by age 4
Pedigree for Tay-Sachs
Simple Dominant Heredity Dominant disorders are inherited as Mendels rule of dominance predicted: Only one dominant allele has to be inherited from either parent.
Disorders: Huntingtons Disease A lethal genetic disorder that causes certain areas of the brain to break down. Does not occur until 30-50 years of age so this is why it can be passed along. There is a genetic test that can test the presence of the allelewould you want to know?
Is it Dominant or Recessive Dominant, only one parent has the disorder. 3 214 5 6 1 1 2 23 4 3 I. III. II.
Is it Dominant or Recessive Recessive, neither parent has the disorder. Both are heterozygous. 3 214 5 6 1 1 2 23 4 3 I. III. II.
Chapter 12 Section 2: When Heredity Rules are Different
Complex Patterns of Heredity Most traits are not simply dominant or recessive Incomplete dominance: when the phenotype of the heterozygous individual is in between those of the two homozygotes (homozygous dominant & homozygous recessive)
Red flower color (RR) is dominant White flower color (rr) is recessive Pink colored flowers (Rr)
Codominace: when the alleles of both homozygotes (BB or WW) are expressed equally in the heterozygous individual If a black chicken (BB) is crossed with a white chicken (WW), all offspring will be checkered Example: sickle-cell anemia
Sex-linked traits: when traits are controlled by genes located on sex chromosomes X-linked disorders: generally passed on from mother to son The genetic abnormality is found on the X chromosome Females are XX, males are XY
If a female has a normal X, it would be dominant over the defective X In males, it will not be masked by a corresponding dominant allele because they have a Y chromosome Ex: hemophilia & Lesch-Nyhan syndrome
Y-linked disorders: only passed on from father to son Examples: excessive hair growth of the ears & male infertility
Polygenic inheritance: when a trait is controlled by many genes Examples: height, eye color, skin color, & blood type
Changes in Chromosomal Numbers Humans have 23 pairs of chromosomes (46 total); more or less = disorder Autosomes: a non-sex chromosome Known as chromosomes 1-22
Sex chromosomes: 23 rd pair in humans that determine a persons sex Example: Downs Syndrome (trisomy 21)
8 Environmental Factors That Can Also Influence Gene Expresssion 1. temperature 2. light 3. nutrition 4. chemicals 5. infectious agents 6. hormones 7. structural differences 8. age *** know examples***