Chapter 16 Flashcards
how does inheritance work?
- inheritance: traits passed parents to the offspring
- Pre-Mendel, blending inheritance was assumed
- blue-eyed parent X green-eyed parent -> gray-eyed child
- OR if one was “stronger”, child would have traits of that parent
inheritance
- we now know that blending inheritance is WRONG
- Mendel- mid 1800s
- conducted experiments, collected data, and used statistics to help explain the results
- he figured out what occurred in the black box
Garden Pea, Pisum sativum
- why did Mendel study peas?
- Reason 1- many readily apparent traits
- either/ or inheritance
- either tall OR short, smooth OR wrinkled
- Reason 2- normally self-fertilizing
- true breeding lines exhibit the same traits
- Reason 3- easy to use
- cross-fertilization or hybridization
Genotype
- genetic (allele) composition of individual
- TT and tt- homozygous
- Tt- heterozygous
- each letter represents one gene on one chromosome
- each pair of letters represents the two copies of a gene on both homologous chromosomes
Phenotype
- characteristics that are the result of gene expression
- TT or Tt are tall
- tt are dwarf
single-factor cross
- experimenter follows the variants of only 1 trait
- monohybrid
P generation
-true-breeding parents
F1 generation
- offspring of P cross
- monohybrids- if parents differ in 1 trait
F2 generation
-F1 self-fertilizes
Traits are dominant and recessive (mendels 3 ideas)
- dominant- displayed in hybrids
- recessive- masked by dominant
Genes and alleles (mendels 3 ideas)
- particulate mechanism of inheritance
- “unit factors” (aka genes)
- every individual has two genes for a character
- gene has 2 variants forms, or alleles
segregation of alleles (mendels 3 ideas)
-two copies of a gene carried by an F1 plant segregate (separate) from each other, so that each sperm or egg carries only one allele
0F2 traits follow approximately 3:1 ratio
Mendels law of segregation
- two copies of a gene segregate from each other during the transmission from parent to offspring
- during sexual reproduction, homologues replicate and pair
- the pairs segregate (Meiosis I)
- one cell has 2 copies of T-> TT
- one cell has 2 copies of t-> tt
- sister chromatids separate (Meiosis II)
- 2 gametes with T and 2 gametes with t
testcross
- dominant phenotype; genotype could be TT or Tt
- multiple by
- recessive phenotype; genotype must be tt
- to determine the genotype of a parent with a dominant phenotype, examine genotypes of offspring when cross with a recessive phenotype
law of independent assortment
- alleles of different genes assort independently of each other during gamete formation
- each trait sorts itself independently of the others
- not always he case- if genes are on the same chromosome
chromosome theory of inheritance
- chromosomes contain the genetic material
- chromosomes are replicated and passed from parent to offspring (cells or kids)
- each chromosome retains its individuality during cell division and gamete formation - each diploid cell contains two sets of chromosomes, which are found in homologous pairs
- one inherited from mom and one from dad
- each chromosome of the the set carries a full complement of the same genes - during meiosis, one of each chromosome pair segregates randomly into the gamete nucleus
- gametes are haploid cells that combine to form a diploid cell during fertilization
- each gamete transmits one set of chromosomes to the offspring
Pedigree analysis
- trait analyzed over multiple generations
- medical pedigrees can be helpful for genetic disorders
- provide a family history
- help establish probabilities for future inheritance
- help determine whether condition is
- dominant or recessive
- X-linked or autosomal
Cystic fibrosis (pedigree analysis)
- autosomal recessive traint
- approximately 3% of Americans with European descent are heterozygous carries of the recessive CF allele and phenotypically normal
- genotype Cc
- individuals who are homozygous recessive exhibit disease systems
- genotype?
- 2 unaffected individuals CAN HAVE an affected child!
- what is the probability?
autosomal dominant disease pedigree- HD
- autosomal dominant usually seen in every generation
- every affected individual has on affected parent
X-linked inheritance
- genes found on the X but not the Y are X-linked
- sex-linked genes: on one sex chromosome but not the other
- in humans X is larger, carries more gene
- 1,500 vs 70 genes
- males are hemizygous for X-linked genes
X-linked inheritance- Color blindness
- affects more males than females
- 8% of the male population has some degree of colorblindness
- 0.5% of the female population has some degree of colorblindness
- females are “protected”
sex-ratio dependent sex determination
- in some fish, sex is controlled dominance heiarchy
- clownfish are all born male
- a dominant male will turn female when the current female of the group dies
mendelian inheritance- variations
- inheritance pattern of genes that segregate and assort independently
- simple Mendelian inheritance- one trait is completely dominant over the other
- phenotypic ratios are predictable via Mendels laws
- occurs because dominant gene is enough for functionality
incomplete dominance
- heterozygote displays intermediate trait
- why is this not “blending”
Phenylketonuria (incomplete dominance)
- autosomal recessive metabolic disorder
- cannot break down phenyalanine
homozygotes (incomplete dominance)
-mental impairment and seizures
heterozygotes (incomplete dominance)
- do not exhibit symptoms
- but 2x amount of phenylalanine in blood
codominance
more than two alleles expressed
Human blood type (codominance)
- human blood types; A, B, AB and O
- determined by antigens on the surface of RBC
- antigens are glycoproteins; used for cell-cell recognition
- coded for single gene on chromosome 9
- each individual has 2 alleles of gene that codes for antigens
- can be identical or non-identical
- 3 alleles of this gene exist in the population
- “I^A” allele-> “A” antigen
- “I^B” allele -> “B” antigen
- “O” allele -> no antigen encoded
determining blood type
- body attacks cells with foreign antigens with antibodies
- individuals with blood type A will not recognized by bodies with blood type B or vice versa
- results in rejection of blood transfusion
- O is universal blood donor (has no antigens)
pleiotropy
- genes can exhibit this
- affects multiple trait
role of environment
- expression of genes is determined by the environment
- genetically identical plants- different heights in different temps
gene-environment interactions
- gene expression can vary depending on environment
- example: hydrangeas
- clones express different colors based on soil conditions
Phenotypic plasticity
the ability of an individual to alter genetic expression in response to the environment
Role of environment; human example
- Phenylkaptonurians can develop normally if given a diet free of phenylalanine
- if their diet contains phenylalanine, they develop mental impairments, underdeveloped teeth and more
