Genes and Inheritance
Hints for analyzing pedigrees
1. If the individual is homozygous recessive, then both parents MUST have at least one recessive allele (parents are heterozygous or homozygous recessive) 2. If an individual shows the dominant trait, then at least one of the parents must have the dominant phenotype. Pretty obvious 3. If both parents are homozygous recessive, then ALL offspring will be homozygous recessive
Mendel's experiments
1. Inheritance is determined by units or factors that are passed on to offspring 2. Individuals inherit one such unit from each parent for a trait 3. A trait may not show up in an individual, but can still be passed on to the next generation
Punnett Square
A tool used to analyze possible breeding outcomes; used to determine all the possible outcomes in offspring when the sex cells unite in fertilization; used to predict probabilities of the genotype and phenotypes of a cross between two organisms; used because alleles separate in meiosis and end up in gametes
Non-Mendelian Inheritance
A type of inheritance that does not follow Mendelian laws of inheritance; incomplete dominance; codominance; multiple alleles; polygenic traits; sex-linked traits; sex-influenced traits; sex-limited traits; epistasis; pleiotropy
Pedigree
A visual chart that depicts a family history or the transmission of a specific trait; tools in determining patterns of inheritance of specific trait; traces autosomal recessive, autosomal dominant, and sex-linked traits; affected individuals are darkened; half way is carrier (sex linked); focuses on specific genetic trait; shows phenotype; first born child appears to the left and last is to the right
sum rule of probability (OR)
Add; the chance of an event that can occur in more than one way;
Recessive allele
An allele that is hidden whenever the dominant allele is present; the allele that is expressed as long as no dominant alleles are present
Trait
An inherited characteristic of an organism
Dominant
An inherited trait which is present even when inherited only from one parent (BB or Bb)
Rh factor
Another antigen system (+/-); named after Rhesus monkeys in which it was first isolated; Rh + or RH - refers to the presence of absence of the D antigen in the RBC; 85% of people are +; blood transfusions: a person with Rh- blood can receive blood only from a donor with Rh- blood; Complications in pregnancy: if Rh- mother is carrying and Rh+ child, mother will produce antibodies that would cross the placenta and attack the fetus (called: Hemoletic Disease of Newborns)
Gregor Mendel
Augustinian monk and botanist whose experiments in breeding garden peas led to his eventual recognition as founder of the science of genetics; discoveries in the field of genetics; 19th century; father of genetics; experiments explained concepts of dominant alleles and recessive alleles; experiments were concluded with 2 laws: the law of segregation and the law of independent assortment
sex-limited traits
Autosomal traits that are only expressed in one sex (turned off in the other sex); Ex: facial hair in women vs. coarse facial hair in men; associated with primary and secondary sex characteristics; visible within one sex; barred coloring In chickens only for roosters
incomplete dominance
Blending of a trait; one allele is not completely dominant over the other; each allele is represented by its own capital letter; you will not see the 3:1 ratio in the F2 generation
Co-dominance
Both alleles are phenotypically expressed when present; each allele is represented by its own capital letter; Ex: sickle cell anemia trait;
Tracing Autosomal recessive trait
Cystic fibrosis, falconi anemia, albinism, phenylketonuria; the dominant is normal, the recessive is one of the previously listed
blending inheritance
Early concept of heredity proposing that offspring possess a mixture of the traits from both parents; pre-Mendel; a model where the offspring would have a mix of characteristics that the parents have; did not explain Mendel's observations
Why pea plants?
Easy to maintain, easy to breed, and reproduce quickly; multiple generations can be observed; grow quickly; have several visible characteristics that exist in two different forms; self pollination (he removed anthers from some pea plants in order to study the offspring of two different parent plants); seed shape and color; flower color; pod shape and color; stem placement and size
Mendel's first set of experiments
Experimented with just one characteristic (ex: flower color) at a time; Steps; 1. Allowed each variety of pea plants to self-pollinate for several generations. Wanted to ensure that the offspring of each variety would display the same characteristics (Ex: purple flowers will only produce purple flowers); these plants formed the P generation 2. Cross pollinated two P generation plants with different characteristics (Ex. A pea plant with purple flowers and a pea plant with white flowers); theses plants formed the F1 generation 3. He allowed the F1 generation plants to self pollinate; the offspring formed the F2 generation R Results: all F1 plants were alike; F2 generation were no longer the same (showed the two different characteristics); F2 phenotypic ration 3:1
Mendel's second set of experiments
Experimented with two characteristics at a time (yellow round seeds or green wrinkled seeds); wanted to know if different characteristics are inherited together(were yellow seeds always round and were green seeds always wrinkled); steps were the same like the ones from the first experiments Results: all F1 were the same; F2 showed all possible combinations of the two characteristics; F2 plants displayed all possible combinations in a consistent ration of 9:3:3:1
sex-linked traits
Genes for these traits are located on the sex chromosomes (X and Y chromosomes); more sex linked genes on the X chromosome bc it is larger than the Y chromosome; usually recessive and on the X chromosome; male determines the gender of the child; Ex: color blindness and hemophilia genes are located on the X chromosome; genes are only located on the X chromosome; always expressed in males when allele is present; females must inherit 2 recessive alleles in order to express the trait; if females only inherit one allele, they are carriers of the trait (X^T X^t); when working with sex linked problems always include sex chromosomes
Linked genes
Genes located on the same chromosome that tend to be inherited together in genetic crosses; exception to independent assortment
Mendel's conclusions
His conclusions formed the basis of genetics: some factor (genes) controls what characteristics are observed; a pair of genes control each characteristic; each parent contributes one copy of the gene; alleles control each characteristic; when an organism inherits two different alleles, only the dominant allele is expressed (recessive one is hidden)
AB... co-dominant
I^A I^B; type AB blood; codes for A and B antigens on the RBC surface; no A or B antibodies are made; universal recipient; donates to AB; receives from A, B,AB, O; 4% frequency
A allele
I^A; type a blood; codes for A antigens on the RBC surface; B antibodies circulate in the bloodstream; donates to A or AB; receives from A or O; 42% frequency
B allele
I^B; type B blood; codes for B antigens in the RBC surface; A antibodies circulate in bloodstream; donates to B or AB; receives from B or O; 10% frequency
Law of Dominance
If both dominant and recessive allele are inherited, the dominant allele will determine the phenotype of the trait; Experiment: cross two purebred parents (TT and tt) and offspring will be tall
Test cross
Make Punnett square to cross an UNKNOWN parent genotype with a homozygous recessive individual for that trait; used to determine the genotype - if ALL offspring show the dominant trait, then the unknown is homozygous dominant - if any (about 1/2) offspring show recessive trait, then the unknown is heterozygous - large number of offspring are needed for reliable results, so not 100% accurate
Law of Segregation
Mendel's first law of inheritance. The law states that there are two factors controlling a given characteristic in which one dominates the other. These factors separate during reproduction as go to different gametes, or a mature reproductive cell that eventually unites with another gamete to reproduce; during gamete formation, 2 alleles for a trait are separated from each other; occurs during meiosis anaphase II; Experiment: Tt x Tt... offspring shows a 3:1 phenotypic ratio and a 1:2:1 genotype ratio
law of independent assortment
Mendel's second law of inheritance. The factors controlling the different characteristics are inherited separately and therefore are not related; when 2 or more traits are inherited, alleles are sorted independently during the production of gametes (metaphase I); Experiment: he noticed that the offspring of his crosses didn't always match the combination of traits in the parents ( dihybrid cross- 9:3:3:1 ratio between offspring of parents heterozygous for two traits) Exception: Linked Genes- can be inherited together bc they are close together on a chromosome
Product rule (and)
Multiply; the probability that two or more independent events will occur is equal to the product of their individual probabilities; can be used to find the frequencies of fertilization events
Pleitropy
One gene has multiple phenotypes (traits) associated with it; Ex: the gene may code for a product that is used in many cells, or for a signaling molecule (hormone) that has a widespread effect; results in positive or negative effects on one or more traits; cystic fibrosis ( a mutation in a single gene affects multiple systems in this case the lungs, pancreas, and sweat glands)and sickle cell disease; Ex: coat color in mice bc Y can be coat color and lethality
Allele
One of the hereditary material for a particular trait; different forms of a specific gene
polygenic traits
One trait is controlled by more than one gene; multiple genes have similar or additive effects on a single trait; many different variations of the phenotype will be observable; Ex: human height, intelligence, skin and eye color; body form; phenotypes graphed result in a Bell curve
P, F1, F2
P- parent generation; purebred trait for two forms (Ex two colors); all purple F1- offspring of P; hétérozygote parents; all Pp; purple F2- offspring of F1 X F1; 3:1 purple:white; 1:2:1 PP:Pp:pp
Tracing Autosomal Dominant trait
Polydactyl, achondroplasia, neurofibromatosis, familial hypercholesterolemia; dominant is one of the above, recessive is normal
Tracing Sex linked traits
Sex linked recessive trait is determined by an allele carried on the X chromosome; If males have one recessive copy of the allele, the will express the trait; females that are heterozygous for a trait are carriers; females that are homozygous recessive for the trait will have the disease; Ex: hemophilia and colorblindness
dominant allele
The allele this is expressed as long as a dominant allele is present
Recessive
The form of the gene that shows up only when inherited from Both parents (bb)
Epistasis
The gene of one gene masks or interferes with the expression of another gene; describes how gene interactions can affect phenotypes; one gene affecting the expression of another
Genotype
The kinds of gene an individual carries; the set of genes in our DNA which is responsible for a particular trait; what your genes say you are
Probability
The likelihood of an occurrence; the allele that each parent will pass on to its offspring is based on....
Phenotype
The observable expression of a trait; the physical expression, characteristic, or trait
Hybrid
The resulting offspring from a cross between two different types of parents
Genetics
The science that shows how characteristics are passed on from one generation to the next; the science of heredity
Epigentics
The scientific field that examines inheritable traits not caused by changes in the DNA code; study of how DNA interacts with a multitude of smaller molecules found within cells which can activate and deactivate genes; epigenome- made up of chemical tags (methyl) and proteins that can attach to DNA ( can turn genes on and off); this change is reversible; can probably be inherited; environmental influences: certain foods, stress, toxins can increase or decrease the methylation (tags); epigenetic changes involve adding or removing methyl tags; adding methyl tags turns genes on and off; epigenetics plays a role in embryonic development, puberty, pregnancy, and cancer; environmental factors can cause changes in your genome by changing the expression of genes in the DNA; as twins age, the pattern of methyl tags on their DNA changes; study of changes in gene expression not due to changes in DNA sequence
Gene
The segment of DNA that determines a particular trait; determines the inheritance of a particular trait; codes for a single protein or RNA; controls what characteristics are expressed; CONSIDERED the units of heredity
Heredity
Transmission of genetic information from parent to offspring; the passing of traits from parent to offspring
Heterozygous
Two different genes for the same trait (Bb)
Homozygous
Two identical genes for the same trait (BB)
monohybrid cross
Used when studying only one trait
dihybrid cross
Used when studying the inheritance of two traits; round and yellow with green and wrinkled; possible gametes are figured out with foil 1. Write the génotypes of each parent 2. Set up 2 Punnett squares with mini crosses of each trait 3. Solve and write the probability of the genotype for EACH trait 4. Use and/or rules to determine the probabilities of genotypes and phenotypes for the combined traits
Sex influenced traits
autosomal traits that are influenced by sex of the individual; expressed in males and females, but influenced by the presence of male and female hormones (estrogen and testosterone); if a male has one recessive allele he will express that trait, but it will take two recessive alleles for a female to show that same trait; Ex: patterned baldness
O allele
ii; type O blood; no glycoproteins/antigens on the RBS surface; A and B antibodies circulating in bloodstream; universal donor; donates to A, B, AB, O; receives from O; 44% frequency
Multiple alleles
many genes have more than two alleles; an individual can only have two alleles, but by increasing the variety of alleles there are to choose from, this increases the variety of different expressions possible; blood types(3 allele choices: A B O) are created by multiple alleles and the alleles also exhibit co-dominance; one letter is chosen to represent the gene and then a superscript letter above it represents the allele; 3 or more forms of a gene that code for a single trait
