Chapter 11 Complex Inheritance and Human Heredity Basic Patterns of Human Inheritance Section 11.1

Basic Patterns of Human Inheritance  The inheritance of a trait over several generations can be shown in a pedigree

Recessive Genetic Disorders  Cystic fibrosis  Albinism  Galactosemia  Tay-sachs disease  Alkaptonuria  Person must be homozygous recessive for disorder to appear  Know table 11.2 on page 297

Recessive Genetic Disorders  Carrier- individual that is heterozygous for the recessive disorder

Dominant Genetic Disorders  Some disorders, are caused by the dominant alleles (Huntington ’ s disease)  The individuals without it are homozygous recessive for the trait  Achondroplasia – most commonly known as dwarfism : 75% of individuals with this disorder are born to parents of average size, causing the condition to be a new mutuation

Pedigrees  Family history  A diagram that traces inheritance of a particular trait through several generations  Uses symbols to illustrate inherited traits Males - represented with squares Females – represented with circles Dark symbol represents expressed trait Light symbol represents masked trait Carrier represented by half and half symbol

Pedigrees  Horizontal lines – parents  Vertical lines – generation  Roman numeral – generation  Arabic numeral – individuals in certain generation  Connected prongs – siblings listed from left to right in birth order

Analyzing Pedigrees  Pedigrees can be used to illustrate the inheritance of a disease  Complete the MiniLab 11.1 on page 300. You will be constructing a pedigree for a hypothetical trait. Have fun!

Inferring genotypes  Pedigrees are used to infer genotypes from the observation of phenotypes  Pedigrees help genetic counselors determine whether inheritance patterns are dominant or recessive  Once inheritance pattern is determined – the genotypes can be determined through pedigree analysis

Inferring genotypes  Dominant traits are easier to recognize than recessive traits since they are exhibited in the phenotype  Once the genotypes are determined, disorders can be predicted

Predicting disorders  If good records are kept within families, disorders in future offspring can be predicted  This is done by evaluating members of the family  The study of human genetics is difficult – scientists are limited by time, ethics and circumstances

Complex Patterns of Inheritance Section 11.2  Complex inheritance of traits does not follow inheritance patterns described by Mendel  Incomplete dominance When there is a blending of traits in the heterozygous state: Rr – pink, if R-red, r-white

Codominance  When both traits are expressed in the heterozygous state: Rr – roan, if R-red and r-white

Codominance

 Sickle-cell anemia – allele responsible is common in people of African descent  Sickle-cell disease affects the red blood cells and their ability to transport oxygen  Changes in hemoglobin causes RBCs to change to a sickle shape and do not transport oxygen because they block circulation in small blood vessels  Those who are heterozygous for the trait have both normal and sickle-shaped cells and can live relatively normally as the normal blood cells compensate for the sickle-shaped cells

Codominance  Sickle-cell disease and malaria – those who are heterozygous for sickle-cell anemia tend to have higher resistance to malaria  Higher malaria : lower sickle-cell  Lower malaria: higher sickle-cell

Multiple Alleles  Not all traits are determined by two alleles.  Multiple alleles – when inheritance is determined by more than two sets of alleles.  Human blood groups are an example: A – I A I A or I A i B - I B I B or I B i AB – I A I B O - ii

Multiple Alleles  ABO blood groups have three forms of alleles called AB markers  Blood type allele (i) is recessive  Blood type alleles (I A ) and (I B ) are codominant  Blood also has Rh factors ( Rh+ or Rh-) Rh+ is dominant Named after Rhesus monkey

Coat color of rabbits  Multiple alleles can demonstrate a hierarchy of dominance  In rabbits, four alleles code for coat color C, c ch, c h, and c C is dominant = full coat color if c ch is present – dominant to other two If c h is present – dominant to c If c is present = recessive to all others resulting in albino Hierarchy: C > c ch > c h > c

Multiple Alleles  The presence of multiple alleles increases the possible number of genotypes and phenotypes  Without multiple allele dominance, two alleles only produce three possible genotypes  Variation in rabbit coat color comes from the interaction of the color gene with other genes

Epistasis  Seen in coat color of Labrador retrievers – results of one allele hiding the effects of another allele  Results in yellow to black coloration

Epistasis  A Labrador ’ s coat color is controlled by two sets of alleles  The dominant allele (E) determines whether the fur will have dark pigment EEbb or Eebb will result in a chocolate lab eeBb or eeBB will produce yellow labs because the e allele masks the B

Epistasis pg. 305

Sex Determination  One pair of the 23 pair of chromosomes a human contains are sex chromosomes which determine the individuals gender  Two types: X and Y  XX – female  XY – male  Other 22 pair of chromosomes are autosomes

Dosage Compensation  X chromosome is larger than Y chromosome  Y chromosome mainly has genes that relate to the development of male characteristics  One X chromosome stops working in the female ’ s body cells – is called dosage compensation or X-inactivation  Occurs in all mammals

Chromosome Inactivation  Calico coat color in cats is caused by the random inactivation of a particular X chromosome

Barr bodies  Only females have Barr bodies in their cell nuclei  Inactivated X chromosomes

Sex-Linked Traits  Genes located on the X chromosome  Because males only have one X chromosome, they are affected by recessive X-linked traits more often than females  Females are less likely to express a recessive X-linked trait because the other X chromosome may mask the effect of the trait

Sex-Linked Traits  Baldness  Male is bald if heterozygous for the trait (dominant)  Female is bald if homozygous recessive for the trait (recessive)

Sex-Linked Traits  Red-green Color Blindness Recessive X-linked trait About 8% of males in US have it

Sex-Linked Traits The sex-linked trait is represented by writing the allele on the X chromosome X B - normal X b – red-green color blind Y – Y chromosome ** Red-green colorblindness is very rare in females***

Hemophilia  Recessive sex-linked disorder  Characterized by delayed clotting of blood  More common in males than females  Queen Victoria ’ s Pedigree – page 308

Polygenic Traits Traits that arise from the interaction of multiple pairs of genes Traits that are polygenic: *skin color *height *eye color *fingerprint pattern

Environmental Influences  Sunlight – if not sufficient in flowering plants, will not bloom  Water - if not sufficient, plants will lose leaves  Temperature – most organisms experience phenotypic changes from extreme temperature changes Also can affect expression of genes – Siamese cat coloring

Twin Studies  Identical twins – genetically the same  Nature vs. nurture – genetic or environmental  Concordance rate – the percentage of twins who both express a given trait A large difference between fraternal twins and identical twins shows a strong genetic influence Figure 11.16