MCAT Study Guide Biology Ch. 6 – Genetics 2017-08-15T06:45:06+00:00



1.     Genetics – science that describes the inheritance of traits from one generation to the next

2.     Gene – fundamental unit of inheritance

3.     Locus – location of a gene on a chromosome

4.     Homologous chromosomes – two nonidentical copies of a chromosome (from mom and dad)

5.     Alleles – different versions of a gene


1.     Genotype – DNA sequence of alleles a person carries

2.     Heterozygote – an individual carrying two different alleles at a given locus

3.     Phenotype – physical expression of the genotype

4.     Incomplete dominance – when the phenotype of a heterozygote is a blend of both alleles

5.     Codominance – similar to above, but the phenotype is not blended; they are both expressed

6.     Pleiotropism – alteration of seemingly unrelated phenotype by the alteration of one gene

7.     Polygenism – one trait that is influence by many genes

8.     Penetrance – the likelihood that a person with a given genotype will express the expected phenotype (can be related to age, lifestyle, environment, etc)

9.     Epistasis – situation where expression of alleles for one gene is dependent on a different gene (curly hair cannot be expressed if the person is bald)


1.     Sex-linked trait – traits that are determined by genes on the X or Y chromosome

II.          6.2:  MEIOSIS


1.     Meiosis – the production of haploid cells such a as gametes from a diploid cell (2 copies → 1)

2.     Spermatogonia and oogonia – specialized cells in males and females that undergo meiosis

3.     Similarities of mitosis and meiosis:

●       Both are preceded by one round of replication of the genome (S phase)

●       Different phases in cell division are referred to by the same names (P, M, A, T)

●       Both are mechanically similar

4.     Differences between meiosis and mitosis:

●       Replicaion of the genome is followed by one round of cell division in mitosis and two rounds in meiosis (meiosis I and II)

●       In meiosis, recombination occurs between homologous chromosomes

5.     Steps in Meiosis I:

●       Prophase I – chromosomes have 2 chromatids, and the homologous pairs align themselves (called bivalent or tetrad); genes are then swapped in process called crossing over or recombination

●       Metaphase I – alignment along the metaphase plate occurs in mitosis; the difference is that the tetrads are aligned together at the center, where in mitosis, sister chromatids are aligned at the center

●       Anaphase I – homologous chromosomes are separated, and sister chromatids remain together

●       Telophase I – cell divides into 2 cells (cells are now haploid but still exist as sister chromatids)

6.     Steps in Meiosis II:

●       Prophase II – no replication happens (already exist as chromatids)

●       Metaphase II – chromatids align at metaphase plate

●       Anaphase II – sister chromatids are separated

●       Telophase II – cell divides


Failure of homologous chromosomes or sister chromatids to separate correctly

1.     Gametes resulting from nondisjunction will have two copies or no copies (resulting in trisomy or monosomy when fusion with another gamete occurs)



1.     Law of Segregation – two alleles of an individual are separated and passed onto the next generation singly

2.     Law of Independent Assortment – the alleles of one gene will separate into gametes independently of alleles for another gene

B.     Definitions:

1.     Cross – mating between plants, used to discern genotypes by looking at phenotypes

2.     Testcross – a way to deduce the genotype by looking at the phenotype of the progeny

3.     F1 Generation – the progeny of the testcross


Visual tool used to predict the results of a cross between two individuals using independent assortment


1.     Rule of Multiplication – the probability of both of two independent events happening can be found by multiplying the odds of either event alone

2.     Rule of Addition – the probability of the chance of either of two independent events happening is the sum of their probabilities minus the probability of both happening together

●       Pa or Pb = Pa + Pb – PaPb

IV.          6.4:  LINKAGE

A.     Linkage

The failure genes to display independent assortment (probably because they are located on the same chromosome)

1.     The closer genes are located on a chromosome, the less likely they are to be independently assorted


1.     Linkage is the exception to the law of independent assortment

2.     Meitotic recombination provides an exception to linkage

3.     Frequency of Recombination – the number of recombinant phenotypes resulting from a cross divided by the total number of progeny:

●       RF = recombination frequency = (number of recombinants)/(total number of offspring)

●       This is directly proportional to the physical distance along linear length of DNA

●       This can be used to map genes in relation to each other on chromosomes


A.     Autosomal recessive

Two copies of the recessive allele are required to confer phenotype

B.     Autosomal dominant

A single copy of the allele will confer the phenotype

C.    Mitochondrial

Very rare traits that are inherited through mitochondrial DNA

1.     Progeny of affected frmales are affected

2.     Genes are given the prefix “mt”

D.    Y-linked

Sex-linked trait that will only be passed from male parent to male children

E.     X-linked recessive

Females are the carriers and male offspring will express these alleles (females must be homozygous recessive to express this trait)

F.     X-linked dominant

More difficult to identify; still tend to affect more males than females



Describes the inheritance traits in populations over time


1.     HW law – the frequencies of alleles in the gene pool of a population will not change over time, provided

●       There is no mutation

●       There is no migration

●       There is no natural selection

●       There is no random mating

●       The population is large enough to prevent random drifts in allele frequencies

2.     Mathematical terms:

●       p2 + 2pq + q2 = 1

●       p + q = 1

3.     HW equilibrium – after one generation, alleles reach equilibrium in which their frequencies do not change


1.     Assumptions are not possible

●       Mutation is inevitable

●       Migration will likely occur, which will disturb equilibrium

●       Natural selection will occur unless there are unlimited resources, no natural selection, no disease, etc

●       Non-random mating does not occur

●       Random drift occurs in small populations


Inheritance Pattern Identification Techniques Unaffected Genotypes Affected Genotypes
Autosomal recessive Can skip generationsAffected males = Affected females AA, Aa aa
Autosomal dominant Does not skip generationsAffected males = affected females

Affected parents pass to half or to all of offspring

aa AA, Aa
Mitochondrial Maternal inheritanceAffected female has all affected children

Affected male cannot pass trait

a A
Y-linked Affects males onlyAll affected fathers have all affected sons XYa XYA
X-linked recessive Can skip generationsAffected males > affected females

Unaffected females can have affected sons, but both affected and unaffected daughters



X-linked dominant Hardest to identifyDoes not skip generations

Usually affected males > affected females

Affected fathers have all affected daughters

Affected mothers can have unaffected offspring, both sexes equally affected






A.     THEORY:

1.     In a population, there are heritable differences between individuals

2.     Heritable traits (alleles) produce traits (phenotypes) that affect the ability of an organism to survive and have offspring

3.     Some individuals have phenotypes that allow them to survive longer, be healthier, and have more offspring than others

4.     Over time, these alleles that lead to more offspring are passed on more frequently while other alleles become less abundant

5.     Changes in the allele frequency are the basis of evolution in species and populations


1.     New alleles

2.     New combinations of existing alleles


1.     Directional selection:  natural selection removes certain alleles at one end of an extreme (ex:  short giraffes die off because they cannot reach the tall food)

2.     Divergent selection:  natural selection removes members in the middle (ex:  small deer can hide, large deer can fight)

3.     Stabilizing selection:  extremes at both ends are selected against (ex:  birds too large or too small cannot mate)

4.     Artificial selection:  humans intervene to alter crops, pets

5.     Sexual selection:  animals develop certain characteristics to select mates

6.     Kin selection:  animals that live socially share alleles and will often sacrifice themselves for other individuals, thus preserving the alleles


A.     Prezygotic barriers to reproduction (prevents formation of a zygote):

1.     Ecological

2.     Mechanical

3.     Behavioral

4.     Gametic

B.     Postzygotic barriers to reproduction

Prevent the development, survival, or reproduction of hybrid animals

C.    Speciation – creation of a new species

1.     Cladogenesis – branching of speciation where one diversifies and becomes two or more new species

2.     Allopatric isolation – cladogenesis initiated by geographic isolation

3.     Anagenesis – one species becomes another by changing so much that if an individual were to go back in time, it would not be able to reproduce with its ancestors

4.     Sympatric speciation – occurs when a species give rise to a new species in the same geographical area, such as through divergent selection

D.    Homologous vs analogous

1.     Homologous structures are similar structures as the result of a common ancestor

2.     Analogous structures serve the same function but are not due to common ancestry

E.     Parallel vs Convergent evolution

1.     Parallel evolution is when 2 species go through similar evolutionary changes due to similar selective pressures (think ice age)

2.     Convergent evolution is when 2 species come to possess many analogous structures due to similar selective pressures (bats and birds)

IX.          6.9:  TAXONOMY

A.     8 Principal Taxonomic Categories:

1.     Domain

2.     Kingdom

3.     Phylum

4.     Class

5.     Order

6.     Family

7.     Genus

8.     Species

●       Dumb Kids Playing On the Freeway Get Squashed

Category Human Characteristics
Domain Eukarya
Kingdom Animalia
Phylum Chordata Possess notochord, dorsal hollow nerve cord, pharyngeal gill slits at some time during embryotic development
Subphylum Vertebrata Bilateral symmetry, cephalization, endoskeleton with vertebral column and four limbs/fins, 2-4 chambered heart and closed circulatory system, respiratory system, excretory system with kidneys, mostly separate sexes
Class Mammalia Hair, 4 limbs, 4-chambered heart, diaphragm for respiration, mammary glands, internal fertilization, some have placental development
Order Primates Well-developed cerebral cortex, opposable thumbs, omnivorous, forward-facing eyes
Family Hominidae Erect posture, intelligence, long period of parental care, cooperation


X.          6.10:  THE ORIGINS OF LIFE

A.     Earth is thought to be 4.5 billion years old

B.     All life evolved from prokaryotes

C.    Early atmosphere was a reducing environment (no O2), where electron donors were prevalent

1.     Oxygen is an electron acceptor and tends to break organic bonds

D.    Simple organic molecules, monomers, formed spontaneously (energy from sun, volcano, lightning)

1.     Laboratory recreations resulted in spontaneous formation of AAs, carbs, lipids, ribonucleotides, etc

2.     Spontaneous polymerization also observed; catalysts thought to be metal ions; this is known as abiotic synthesis (polypeptides made this way are proteinoids)

E.     Proteinoids in water spontaneously formed droplets called microspheres; when lipids are added, lipsomes form, with the lipids forming a layer on the surface of proteins

F.     Coacervates are more complex particles that include polypeptides, nucleic acids, polysaccharides

1.     Coacervates made with pre-existing enzymes are capable of catalyzine reactions

G.    Protobionts

Coacervates, microspheres, and liposomes

1.     These can grow and when they become too large, split in half


Chapter 6 Summary

  • Organisms express phenotypes according to their genotypes
  • Classical dominance occurs when a phenotype or trait is determined by one gene with two alleles, and one allele is dominant and the other is recessive
    • There are several exceptions to classical dominance, including incomplete dominance, codominance, epistasis, pleiotroptism, polygenism, and penetrance
  • Incomplete dominance occurs when two differen alleles for a single trait result in a blended phenotyp
  • Codominance occurs when two different alleles for a single trait are expressed simultaneously, but independently (no blending)
  • Epistasis occurs when the expression of one gene depends on the expression of another
  • Pleiotropic genes affect many different aspects of the overall phenotype, whily polygenic traits are affected by many different genes
  • Penetrance refers to the likelihood that a particular genotype will result in a given phenotype; can be affected by several factors, including age, environment, lifestyle
  • From a single diploid precursor cell, meiosis generates four haploid cells with a random mix of alleles; this is due to crossing over in prophase I and separation of homologous chromosomes in anaphase I
  • Nondisjunction is a failure to separate the DNA properly during meiosis and can result in gametes with improper numbers of chromosomes
  • The Punnett square or the rules of probability can be used to determine the genotypes and phenotypes of offspring from given crosses, or the probability of having offspring with certain traits
  • The rule of multiplication states that the probability of A and B occurring is equal to the probability of A multiplied by the probability of B
  • The rule of addition states that the probability of A or B occurring is equal to the probability of A plus the probability of B, minus the probabililty of A and B occurring together
  • Linkage occurs when 2 gametes are close together on the same chromosome; it leads to alleles being inherited together (less recombination) instead of independently
  • Pedigrees can be used to analyze the patterns of inheritance of different traits
  • The 6 primary modes of inheritance include autosomal recessive, autosomal dominant, mitochondrial, x-linked recessive, x-linked dominant, and y-linked
  • The Hardy-Weinberg law can be used to study population genetics; it assumes classical dominance with only 2 alleles and unchanging allele frequencies; assumptions include:
    • no mutation
    • no natural selection
    • no migration
    • large populations
    • totally random mating
  • Natural selection drives evolution by allowing individuals with random, beneficial mutations to survive and pass those beneficial mutations on to their offspring
  • Homologous structures are the result of divergent evolution to form new species
  • Analogous structures are the result of convergent evolution, in which different species must meet similar environmental challenges

MCAT Study Guide Biology - Kim Matsumoto

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