AP Biology Beyond Mendel’s Laws of Inheritance

AP Biology Incomplete Dominance Heterozygote shows an intermediate, blended phenotype. Example:  RR = red flowers  rr = white flowers  Rr = pink flowers  make 50% less color RR rrRr

AP Biology Codominance  There are two or more alleles that are dominant in a phenotype.  Both alleles are expressed in heterozygous condition.  Ex: Red – RR, white – rr, red and white - Rr

AP Biology Multiple Alleles  Two alleles affect the phenotype equally and separately.  NOT blended phenotype.  Example: human ABO blood groups  3 alleles: I A, I B, i  I A & I B alleles are co-dominant  glycoprotein antigens on RBC  I A I B = both antigens are produced  i allele recessive to both Type A IAIAIAiIAIAIAi Type B IBIBIBiIBIBIBi Type ABIAIBIAIB Type Oii

AP Biology Polygenic Inheritance  Some phenotypes determined by additive effects of 2 or more genes on a single characteristic.  Phenotypes on a continuum.  Example: human traits  skin color  height  weight

AP Biology Sex Linked Traits  Sex Determination in Humans  XX is female.  XY is male.  Every new born has a 50% chance of being female and a 50% chance being male. XX XXX YXY

AP Biology Sex-linked Traits  Traits carried on the sex chromosomes  Mostly on X-Chromosome.  Leads to an unequal occurrence of the trait in the sexes.  Males are more likely to have the recessive trait.  Females can be carriers of recessive allele.  Example: color-blindness in humans (recessive allele represented by a lower case c). Normal vision Heterozygous (carrier) Colorblind Female: XX XCXCXCXC XCXcXCXc XcXcXcXc Male: XY XCYXCYNoneXcYXcY

AP Biology Sex-linked: Hemophilia Hh x HH XHYXHY XHXhXHXh XHXhXHXh XHXH XhXh XHYXHY Y XHXH sex-linked recessive XHXH Y male / sperm XHXH XhXh female / eggs XHXHXHXH XHXhXHXh XHYXHYXhYXhY XHXHXHXH XHYXHY XHXhXHXh XhYXhY carrierdisease

AP Biology Sex-linked Traits Practice Cross a female who does not have hemophilia and is not a carrier with a male with hemophilia.

AP Biology Environmental Effects Phenotype is controlled by both environment & genes Color of Hydrangea flowers is influenced by soil pH. Human skin color is influenced by both genetics and environmental conditions. Coat color in arctic fox influenced by heat sensitive alleles.

AP Biology Karyotypes  A karyotype is a photograph of all of an organism’s chromosomes.  Chromosomes arranged in homologous pairs according to size.  A karyotype gives information about the number of chromosomes, the structure of chromosomes and the sex of the individual. ***Karyotypes help determine chromosome abnormalities (genetic disorders).

AP Biology Human Karyotypes  A normal human karyotype will contain 46 chromosomes (23 pairs).  The first 22 pairs are homologous chromosomes (autosomal).  The 23rd pair contains the sex chromosomes and determines male or female.

AP Biology Down syndrome  Trisomy 21  3 copies of chromosome 21.  1 in 700 children born in U.S.  Chromosome 21 is the smallest human chromosome.  But still causes severe effects.  Frequency of Down syndrome correlates with the age of the mother.

AP Biology Down Syndrome & Age of Mother Mother’s age Incidence of Down Syndrome Under 30<1 in in in in in in in in in in in in in 12 Rate of miscarriage due to amniocentesis:  1970s data 0.5%, or 1 in 200 pregnancies  2006 data <0.1%, or 1 in 1600 pregnancies

AP Biology Sex Chromosomes Abnormalities  Human development more tolerant of wrong numbers in sex chromosome.  But produces a variety of distinct syndromes in humans.  XXY = Klinefelter’s syndrome male  XXX = Trisomy X female  XYY = Jacob’s syndrome male  XO = Turner syndrome female

AP Biology Pedigree Analysis  A pedigree is a family tree that describes the interrelationships of parents and children across generations.  Inheritance patterns of particular traits can be traced and described using pedigrees.

AP Biology  Autosomal Recessive Traits  Rare in pedigree.  Skips generations.  Affects males and females.  Examples: Tay-Sachs disease  Autosomal Dominant Traits  Common in pedigree.  Found in every generation.  Affects males and females.  Example: Achondroplasia (skeletal abnormality causing dwarfism).

AP Biology  X-linked Recessive Trait  Rare in pedigree.  Skips generations.  Males are more often affected.  Affected fathers DO NOT pass to their sons.  X-linked Dominant Trait  Common in pedigree.  Affected father pass to ALL of their daughters.  Males and females equally affected.