Chapter 29: Heredity Flashcards
Genetics
study of the mechanism of hereditary
Human Genome Project (1990-2003)
has determined human DNA sequence, which can aid in genetic research and genetic screening
Diploid number of chromosomes
Diploid number= 46 (23 pairs of homologous chromosomes)= 2n
- In all cells except gametes -
- > Haploid number= 1n
1 pair of sex chromosomes determines the genetic sex
XX= female XY= male
22 pairs of autosomes guide the expression of most other traits
TRUE
gene pairs (alleles)
- alleles are genes that occur at same LOCUS (location) on homologous chromosomes
- homozygous: alleles controlling a single trait are the same (TT, tt)
- heterozygous: alleles for a trait are different (Tt)
- dominant: an allele that masks or suppresses its (recessive) partner
Genetics- gregor mendel
- Austrian monk (1822)
- Teacher, in charge of monastery garden
- Two types of pea plants – tall and short
- Self pollinating vs cross pollinating
*cross pollinating a tall and short plant produces all tall plants, but next generation will produce 3 tall and 1 short
genotype
the genetic makeup (Tt)
phenotype
the way the genotype is expressed (tall pea plant)
sexual sources of genetic variation
- chromosome segregation and independent assortment
- crossover of homologous
- random fertilization of eggs by sperm
- segregation and independent assortment
- > Independent assortment: during gametogenesis, maternal and paternal chromosomes are randomly distributed to daughter cells, (which allele a gamete recieves for gene A has no bearing on the allele it recieves for gene B)
- occurs during metaphase of meiosis
-> segregation: distribution of 2 alleles for a trait to different gametes during meiosis
independent assortment
during gametogenesis, maternal and paternal chromosomes are randomly distributed to daughter cells,
*occurs during metaphase of meiosis
segregation
distribution of 2 alleles for a trait to different gametes during meiosis
segregation and independent assortment
-the number of gamete types= 2^n, where n is the number of homologous pairs
ex: 2^3= 8 (2x2x2)
in a man testes, 2^n= 2^23= 8.5 million
- crossover and genetic recombination
- > genes on the same chromosome are linked
- > chromosomes can cross over, forming a chiasma, and exchange segments
- > crossover occurs during prophase of meiosis
- > recombinant chromosomes have mixed contributions from each parent
karyotype
diploid chromosomal complement displayed in homologous pairs
homologous chromosomes synapse during prophase of meiosis 1. Each chromosome consists of 2 sister chromatids
true
review slides 14-17
random fertilization
adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)
types of inheritance
- most traits are determined by multiple alleles or by the interaction of several gene pairs
- dominant-recessive inheritance
- multiple-allele inheritance
- polygene inheritance
dominant recessive inheritance
- reflects the interaction of dominant and recessive alleles
- punnett square: predicts the possible gene combinations resulting from the mating of parents of known genotypes
traits:
- attached earlobes (unattached= dominant)
- roll tongue
- dimples
- freckles
- curly hair
- cleft chin
- widows peak
dominant disorders
are uncommon because many are lethal and result in death before reproductive age
**exception: Huntington’s Disease is caused by a delayed action gene- person survives long enough to reproduce
dominant recessive inheritance
- most genetic disorders are inherited as simple recessive traits…. albinism, cystic fibrosis, and tay-sachs disease
- heterozygotes are carriers who do not express the trait but can pass it on to their offspring
incomplete dominance
-heterozygous individuals have an intermediate phenotype
- example: sickling gene
- SS= normal Hb is made
- Ss= sickle cell trait (both aberrent and normal Hb are made); can suffer a sickle-cell crisis under prolonged reduction in blood O2
- ss= sickle cell anemia (only aberrant Hb is made; more susceptible to sickle cell crisis)
multiple allele inheritance
- genes that exhibit more than 2 allele forms
- ABO blood grouping is an example
- 3 alleles (I^A, I^B, i) determine the ABO blood type in humans
I^A and I^B are codominant (both are expressed if present), and i is recessive
sex-linked inheritance
- inherited traits determined by genes on the sex chromosomes
- X chromosomes bear over 1400 genes (many for brain function); Y chromosomes carry about 200 genes
- more than 100 sex-linked disorders have been mapped to the X chromosome
sex linked inheritance
X-linked genes are
- found only on the X chromosome
- typically passed from mothers to sons (e.g hemophilia or red-green color blindness)
***males have just 1 X chromosome, thus all X-linked alleles are expressed in males, even if recessive
why are most calico cats female?
Coat color is determined by X chromosome
One X has allele for black spots
One X has allele for orange spots
Female cats can have a combination of black and orange spots, males (only one X) can have only one color
polygene inheritance
- depends on several different gene pairs at different locations acting in tandem
- results in continuous phenotypic variation between 2 extremes
- ex: skin color, eye color, height, metabolic rate, and intelligence
- Skin color is controlled by 3 separately inherited genes, each existing in 2 allelic forms: A, a; B, b: C, c
chromosomal disorders
- if 2 copies of an autosomal chromosome fail to separate during meiosis, and individual may be born with 3 copies of a chromosome
- down syndrome: 3 copies of chromosome 21
phenocopies
- > genotype: (excluding mutations) is unchanging
- > phenotype: can be molded or changed (clay)
- > phenocopies: environmentally produced phenotypes that mimic conditions caused by genetic mutations during embryonic development…. thalidomide babies
- > environmental factors can influence genetic expression after birth
- poor nutrition can affect brain growth, body development, and height
- childhood hormonal deficits can lead to abnormal skeletal growth and proportions
- getting a tan
Beyond DNA: regulation of gene expression
3 levels of control are found in human genome
- > first layer: protein coding genes
- involves less than 2% of a cells DNA
- DNA that is blueprint for protein synthesis
- > second layer: small RNAs
- found in non-protein-coding DNA
- > third layer: epigenetic marks
- stored in proteins and chemical groups that bind to DNA and in way chromatin packaged
small RNAs
microRNAs (miRNAs) and short interfering RNAs (siRNAs)
- act directly on DNA , other RNAs, or proteins
- may silence genes or prevent their expression and appear to play a role in directing apoptosis during development
-> in future, RNA- interfering drugs may treat diseases such as age-related macular degeneration and Parkinson’s disease
Epigenetic marks
- info stored in the proteins and chemical groups bound to DNA
- determine whether DNA is available for transcription or silenced. ‘if DNA is like the alphabet then epigenetic marks are like punctuation’
- Epigenetics: study of heritable changes in gene expression. Change in phenotype without change in genotype
- lifestyle can affect individual epigenetics:
- pollution
- diet
extranuclear (mitochondrial) inheritance
- not all DNA is located in cell’s nucleus
- mitochondria contain 37 of their own genes, referred to as mitochondrial DNA (mtDNA)
- mitochondria are transmitted to embryo by mother in cytoplasm of egg
- errors in mtDNA are linked to rare disorders
- usually problems associated with oxidative phosphorylation (cellular respiration)
- some muscle and neurological problems, possibly alzheimers and parkinsons
genetic screening, counseling, and therapy
-newborn infants are routinely screened for a number of genetic disorders: congenital hip dysplasia, imperforate anus, and other metabolic disorders
other examples:
- screening adult children of parents with huntington’s disease
- testing a woman pregnant for the first time after age 35 to see if the baby has trisomy-21 (down syndrome)
carrier recognition
- Two major avenues for identifying carriers of genes: pedigrees and blood tests
- Pedigrees trace a particular genetic trait through several generations; helps to predict the future
- Blood tests and DNA probes can detect the presence of unexpressed recessive genes
- Tay-Sachs and Cystic Fibrosis genes can be identified by such tests
fetal testing
- Used when there is a known risk of a genetic disorder
- Amniocentesis: amniotic fluid is withdrawn after the 14th week and fluid and cells are examined for genetic abnormalities
- Chorionic villus sampling (CVS): chorionic villi are sampled and karyotyped for genetic abnormalities
human gene therapy
- Genetic engineering has the potential to replace a defective gene
- Defective cells can be infected with a genetically engineered virus containing a functional gene (because they can enter the nucleus)
- The patient’s cells can be directly injected with “corrected” DNA
